The Collaborative International Dictionary of English v.0.48:

Plea \Plea\, n. [OE. plee, plai, plait, fr. OF. plait, plaid,
   plet, LL. placitum judgment, decision, assembly, court, fr.
   L. placitum that which is pleasing, an opinion, sentiment,
   from placere to please. See Please, and cf. Placit,
   Plead.]
   1. (Law) That which is alleged by a party in support of his
      cause; in a stricter sense, an allegation of fact in a
      cause, as distinguished from a demurrer; in a still more
      limited sense, and in modern practice, the defendant's
      answer to the plaintiff's declaration and demand. That
      which the plaintiff alleges in his declaration is answered
      and repelled or justified by the defendant's plea. In
      chancery practice, a plea is a special answer showing or
      relying upon one or more things as a cause why the suit
      should be either dismissed, delayed, or barred. In
      criminal practice, the plea is the defendant's formal
      answer to the indictment or information presented against
      him.
      [1913 Webster]

   2. (Law) A cause in court; a lawsuit; as, the Court of Common
      Pleas. See under Common.
      [1913 Webster]

            The Supreme Judicial Court shall have cognizance of
            pleas real, personal, and mixed.      --Laws of
                                                  Massachusetts.
      [1913 Webster]

   3. That which is alleged or pleaded, in defense or in
      justification; an excuse; an apology. "Necessity, the
      tyrant's plea." --Milton.
      [1913 Webster]

            No plea must serve; 't is cruelty to spare.
                                                  --Denham.
      [1913 Webster]

   4. An urgent prayer or entreaty.
      [1913 Webster]

   Pleas of the crown (Eng. Law), criminal actions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plea
    n 1: a humble request for help from someone in authority [syn:
         supplication, plea]
    2: (law) a defendant's answer by a factual matter (as
       distinguished from a demurrer)
    3: an answer indicating why a suit should be dismissed

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "plea":
   adjuration, alibi, answer, apology, appeal, application, argument,
   argumentum, beseechment, bid, call, case, clamor, cons,
   consideration, counterstatement, cry, defence, defense, demurrer,
   denial, elenchus, entreaty, exception, excuse, explanation,
   extenuation, ignoratio elenchi, imploration, imploring,
   imprecation, invocation, invocatory plea, justification,
   mitigation, objection, obsecration, obtestation, out, overture,
   palliation, petition, plaidoyer, pleading, pleadings, prayer,
   pretext, pros, pros and cons, reason, rebuttal, refutation, reply,
   request, response, right, riposte, rogation, solicitation,
   special demurrer, special pleading, statement of defense, suit,
   supplication, talking point, vindication



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PLEA, practice. The defendant's answer by matter of fact, to the plaintiff's
declaration.
     2. It is distinguished from a demurrer, which opposes matter of law to
the declaration. Steph. Pl. 62.
     3. Pleas are divided into plea dilatory and peremptory; and this is the
most general division to which they are subject.
     4. Subordinate to this is another division; they are either to the
jurisdiction of the court, in suspension of the action; in abatement of the
writ; or, in bar of the action; the first three of which belong to the
dilatory class, the last is of the peremptory kind. Steph. Pl. 63; 1 Chit.
Pl. 425; Lawes, Pl. 36.
     5. The law has prescribed and settled the order of pleading, which the
defendant is to pursue, to wit; 1st. To the jurisdiction of the court. 2d.
To the disability, &c. of the person. 1st. Of the plaintiff. 2d. Of the
defendant. 3d. To the count or declaration. 4th. To the writ. 1st. To the
form of the writ; first, Matter apparent on the face of it, secondly, Matter
dehors. 2d. To the action of the writ. 5th. To the action itself in bar.
     6. This is said to be the natural order of pleading, because each
subsequent, plea admits that there is no foundation for the former. Such is
the English law. 1 Ch. Plead. 425. The rule is different with regard to the
plea of jurisdiction in the courts of the United States and those of
Pennsylvania. 1. Binn. 138; ld. 219; 2 Dall. 368; 3 Dall. 19; 10 S. & R.
229.
     7.-2. Plea, in its ancient sense, means suit or action, and it is
sometimes still used in that sense; for example, A B was summoned to answer
C D of a plea that he render, &c. Steph. Pl. 38, 39, u. 9; Warr. Law
Studies, 272, note n.
     8.-3. This variable word, to plead, has still another and more
popular use, importing forensic argument in a cause, but it is not so
employed by the profession. Steph. Pl. App. note 1.
     9. There are various sorts of pleas, the principal of which are given
below.
    10. Plea in abatement, is when, for any default, the defendant prays
that the writ or plaint do abate, that is, cease against him for that time.
Com. Dig. Abatement, B.
    11. Hence it may be observed, 1st. That the defendant may plead in
Abatement for faults apparent on the writ or plaint itself, or for such as
are shown dehors, or out of the writ or plaint. 2d. That a plea in,
abatement is never perpetual, but only a temporary plea, in form at least,
and if the cause revived, the plaintiff may sue again.
    12. If the defendant plead a plea in abatement, in his plea, he ought
generally to give a better writ to the plaintiff, that is, show him what
other and better writ can be adopted; Com. Dig. Abatement, I 1; but if the
plea go to the matter and substance of the writ, &c., he need not give the
plaintiff another writ. Nor need he do so when the plea avoids the whole
cause of the action. Id. I 2.
    13. Pleas in abatement are divided into those relating, first, to the
disability of the plaintiff or defendant; secondly, to the count or
declaration; thirdly, to the writ. 1 Chit. Pl. 435.
    14.-1. Plea in abatement to the person of the plaintiff. Pleas of this
kind are either that the plaintiff is not in existence, being only a
fictitious person, or dead; or else, that being in existence, he is under
some disability to bring or maintain the action, as by being an alien enemy;
Com. Dig. Abatement, E 4 Bac. Abr. Abatement, B 3; 1 Chit. Pl. 436; or the
plaintiff is a married woman, and she sues alone. See 3 T. R. 631; 6 T. R.
265.
    15. Plea in abatement to the person of the defendant. These pleas are
coverture, and, in the English law, infancy, when the parol shall demur.
When a feme covert is sued, and the objection is merely that the husband
ought to have been sued jointly with her; as when, since entering into the
contract, or committing the tort, she has married; she must, when sued
alone, plead her coverture in abatement, and aver that her husband is
living. 3 T. R. 627; 1 Chit. Pl. 437, 8.
    16.-2. Plea in abatement to the count. Pleas of this kind are for some
uncertainty, repugnancy, or want of form, not appearing on the face of the
writ itself, but apparent from the recital of it in the declaration only; or
else for some variance between the writ and declaration. But it was always
necessary to obtain oyer of the writ before the pleading of these pleas; and
since oyer cannot now be had of the original writ for the purpose of
pleading them, it seems that they can no longer be pleaded. See Oyer.
    17. Plea in abatement to the form of the writ. Such pleas are for some
apparent uncertainty, repugnancy, or want of form, variance from the record,
specialty, &c., mentioned therein, or misnomer of the plaintiff or
defendant. Lawes' Civ. Pl. 106; 1 Chit. Pl. 440.
    18. Plea in abatement to the action of the writ. Pleas of this kind are
pleaded when the action is misconceived, or was prematurely commenced before
the cause of action arose; or when there is another action depending for the
same cause. Tidd's Pr. 579. But as these matters are ground for demurrer or
nonsuit, it is now very unusual to plead them in abatement. See 2 Saund.
210, a.
    19. Plea in avoidance, is one which confesses the matters contained in
the declaration, and avoids the effect of them, by some new matter which
shows that the plaintiff is not entitled to maintain his action. For
example, the plea may admit the contract declared upon, and show that it was
void or voidable, because of the inability of one of the parties to make it,
on account of coverture, infancy, or the like. Lawes, Pl. 122.
    20. Plea in bar, is one that denies that the plaintiff has any cause of
action. 1 Ch. Pl. 459 Co. Litt. 303 b; 6 Co. 7. Or it is one which shows
some ground for barring or defeating the action; and makes prayer to that
effect, Steph. Pl. 70; Britton, 92. See Bar.
    21. A plea in bar is, therefore, distinguished from all pleas of the
dilatory class, as impugning the right of the action altogether, instead of
merely tending to divert the proceedings to another jurisdiction, or suspend
them, or abate the particular writ. It is in short a substantial and
conclusive answer to the action. It follows, from this property, that in
general, it must either deny all, or some essential part of the averments of
fact in the declaration; or, admitting them to be true, allege new facts,
which obviate and repel their legal effect. In the first case the defendant
is said, in the language of pleading, to traverse the matter of the
declaration; in the latter, to confess and avoid it. Pleas in bar are
consequently divided into pleas by way of traverse, and pleas by way of
confession and avoidance. Steph. Pl. 70, 71.
    22. Pleas in bar are, also divided into general or special. General
pleas in bar deny or take issue either upon the whole or part of the
declaration, or contain some new matter which is relied upon by the
defendant in his defence. Lawes Pl. 110.
    23. Special pleas in bar are very various, according to the
circumstances of the defendant's case; as, in personal actions, the
defendant may plead any special matter in denial, avoidance, discharge,
excuse, or justification of the matter alleged in the declaration, which
destroys or bars the plaintiff's action; or he may plead any matter which
estops, or precludes him from averring or insisting on any matter relied
upon by the plaintiff in his declaration. The latter sort of pleas are
called pleas in estoppel. In real actions, the tenant may plead any matter
which destroys and bars the demandant's title; as, a general release. Id.
115, 116.
    24. The general qualities of a plea in bar are, 1. That it be adapted to
the nature and form of the action, and also conformable to the count. Co.
Litt. 303, a 285, b; Bac. Abr. Pleas, I; 1 Roll. Rep. 216.
     2. That it answers all it assumes to answer, and no more. Co. Litt. 303
a; Com. Dig. Pleader, E 1, 36; 1 Saund. 28, n. 1, 2, 3; 2 Bos. & Pull. 427;
3 Bos. & Pull. 174.
     3. In the case of a special plea, that it confess and admit the fact. 3
T. R. 298; 1 Salk. 394; Carth. 380; 1 Saund. 28, n. and 14 u. 3 10 Johns. R.
289.
     4. That it be single. Co. Litt. 304; Bac. Ab. Pleas, 2 Saund. K, 1, 2;
Com Dig. Plead. E 2; 49, 50; Plowd. Com. 140, d.
     5. That it be certain. Com. Dig. Pleader, E 5, 7, 8, 9, 10, 11; C 41;
this Dict. Certainty; Pleading.
     6. It must be direct, positive, and not argumentative. See 6 Cranch,
126; 9 Johns. It. 313.
     7. It must be capable of trial. 8. It must be true and capable of
proof. See Plea, sham.
    25. The parts of a plea in bar may be considered with reference to,
     1. The title of the court in which it is pleaded.
     2. The title of the term.
     3. The names of the parties in the margin. These, however, do not
constitute any part of the plea. The surnames only are usually inserted, and
that of the defendant precedes the plaintiff's; as, "Roeats. Doe."
     4. The commencement which includes the statement of, 1. The name of the
defendant; 2. The appearance; 3. The defence; see Defence; 4. The actio non;
 see Actio non.
     5. The body, which may contain, 1. The inducement; 2. The protestation;
3. Ground of defence 4. Qua est eadem; 5. The traverse.
     6. The conclusion.
    26. Dilatory pleas are such as delay the plaintiff's remedy, by
questioning, not the cause of action, but the propriety of the suit, or the
mode in which the remedy is sought.
    27. Dilatory pleas are divided by Sir William Blackstone, into three
kinds: 1. Pleas to the jurisdiction of the court; as, that the cause of
action arose out of the limits of the jurisdiction of the court, when the
action is local. 2. Pleas to the disability of the plaintiff, or, as they
are usually termed, to' the person of the plaintiff; as, that he is an alien
enemy. 3. Pleas in abatement of the writ, or count; these are founded upon
some defect or mistake, either in the writ itself; as, that the defendant is
misnamed in it, or the like; or in the mode in which the count pursues it;
as, that there is some variance or repugnancy between the count and writ; in
which case, the fault in the count furnishes a cause for abating the writ. 2
Bl. Com. 301 Com. Dig. Abatement, G 1, 8; Id. Pleader, C 14, 15; Bac. Ab.
Pleas, F 7.
    28. All dilatory pleas are sometimes called pleas in abatement, as
contradistinguished to pleas to the action; this is perhaps not strictly
proper, because, though all pleas in abatement are dilatory pleas, yet all
dilatory pleas are not pleas in abatement. Gould on Pl. ch. 2, Sec. 35; vide
1 Chit. PI, ch. 6; Bac. Ab. Abatement, 0; 1 Mass 358; 1 John. Cas. 101. 2. A
plea in discharge, as distinguish ed from a plea in avoidance, is one which
admits the demand, and instead of avoiding the payment or satisfaction of
it, shows that it has been discharged by some matter of fact. Such are pleas
of payment, release, and the like.
    30. A plea in excuse, is one which admits the demand or complaint stated
in the declaration, but excuses the non-compliance of the plaintiff's claim,
or the commission of the act of which he complains, on account of the
defendant having done all in his power to satisfy the former, or not having
teen the culpable author of the latter. A plea of tender is an example of
the former, and a plea of son assault demesne, an instance of the latter.
    31. A foreign plea is one which takes the cause out of the court where
it is pleaded, by showing a want of jurisdiction in that court. 2 Lill. Pr.
Beg. 374; Carth. 402. See the form of the plea in Lill. Ent. 475.
    32. A plea of justification is one in which the defendant professes
purposely to have done the acts which are the subject of the plaintiff's
suit, in order to exercise that right which he considers he might in point
of law exercise, and in the exercise of which he conceives himself not
merely excused, but justified.
    33. A plea puis darrein continuance. Under the ancient law, there were
continuances, i. e. adjournments of the proceedings for certain purposes,
from one day or one term to another; and, in such cases, there was an entry
made on the record, expressing the ground of the adjournment, and appointing
the parties to reappear at a given day.
    34. In the interval between such continuance and the day appointed, the
parties were of course out of court, and consequently not in a situation to
plead. But it sometimes happened, that after a plea had been pleaded, and
while the parties were out of court, in consequence of such continuance, a
new matter of defence arose, which did not exist, and which the defendant
had consequently no opportunity to plead, before the last continuance. This
new defence he was therefore entitled, at the day given for his
reappearance, to plead as a matter that had happened after the last
continuance, puis darrein continuance. In the same cases that occasioned a
continuance in the ancient common law, but in no other, a continuance shall
take place. At the time indeed, when the pleadings are filed and delivered,
no record exists, and there is, therefore, no entry at that time, made on
the record, of the award of a continuance; but the parties are, from the day
when, by the ancient practice, a continuance would have been entered,
supposed to be out of court, and the pleading is suspended, till the day
arrives to which, by the ancient, practice, the continuance would extend. At
that day, the defendant is entitled, if any new matter of defence has arisen
in the interval, to plead it according to the ancient plan, puis darrein
continuance.
    35. A plea puis darrein continuance is not a departure from, but is a
waiver of the first plea, and is always headed by way of substitution for
it, on which no proceeding is afterwards had. 1 Salk. 178; 2 Stran. 1195
Hob. 81; 4 Serg. & Rawle, 239. Great certainty is requisite in pleas of this
description. Doct. Pl. 297; Yelv. 141; Cro. Jac. 261; Freem. 112; 2 Lutw.
1143; 2 Salk. 519; 2 Wils. 139; Co. Entr. 517 b. It is not sufficient to say
generally that after the last continuance such a thing happened, but the day
of the continuance must be shown, and also the time and place must be
alleged where the matter of defence arose. Id. ibid.; Bull. N. P. 309.
    36. Pleas puis darrein continuance are either in bar or abatement; Com.
Dig. Abatement, I 24; and are followed, like other pleas, by a replication
and other pleadings, till issue is attained upon them such pleas must be
verified on oath before they are allowed. 2 Smith's R. 396; Freem. 352; 1
Strange, 493.
    37. A sham plea is one which is known to the pleader to be false, and is
entered for the purpose of delay. There are certain pleas of this kind,
which, in consequence of their having been long and frequently used in
practice, have obtained toleration from the courts; and, though discouraged,
are tacitly allowed; as, for example, the common plea of judgment recovered,
that is, that judgment has been already recovered by the plaintiff, for the
same cause of action. Steph. on Pleading, 444, 445; 1 Chit. Pl. 505, 506.
    38. Plea in suspension of the action. Such a plea is one which shows
some ground for not proceeding in the suit at the present period, and prays
that the pleading may be stayed, until that ground be removed. The number of
these pleas is small. Among them is that which is founded on the nonage of
the parties, and termed parol demurrer. Stephen on Pleading, 64. See,
generally, Bac. Abr. Pleas, Q; Com. Dig. Abatement, I 24, 34; Doct, Pl. 297;
Bull. N. P. 309; Lawes Civ. Pl. 173; 1 Chit. Pl. 634,; Steph. Pl. 81; Bouv.
Inst. Index.



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PLEA, chancery practice. "A plea," says Lord Bacon, speaking of proceedings
in courts of equity, "is a foreign matter to discharge or stay the suit."
Ord. Chan. (ed. Beam.) p. 26. Lord Redesdale defines it to be "a special
answer showing or relying upon one or more things as a cause why the suit
should be either dismissed, delayed or barred." Mitf. Tr. Ch. 177; see Coop.
Eq. Pl. 223; Beames' Pl. Eq. 1. A plea is a special answer to a bill, and
differs in this from an answer in the common form, as it demands the
judgment of the court in the first instance, whether the matter urged by it
does not debar the plaintiff from his title to that answer which the bill
requires. 2  Sch. & Lef. 721.
     2. Pleas are of three sorts: 1. To the jurisdiction of the court. 2. To
the person of the plaintiff. 3. In bar of the plaintiff's suit. Blake's Ch.
Pr. 112. See, generally, Beames' Elem. of Pleas in Eq.; Mitf. Tr. Cha. oh.
2, s. 2, pt. 2; Coop. Eq. Pl. ch. 5; 2 Madd. Ch. Pr. 296 to 331; Blake's Ch.
Pr. 112 to 114; Bouv. Inst. Index, h.t.