VERB (4)

1. appeal or request earnestly;
- Example: "I pleaded with him to stop"

2. offer as an excuse or plea;
- Example: "She was pleading insanity"

3. enter a plea, as in courts of law;
- Example: "She pleaded not guilty"

4. make an allegation in an action or other legal proceeding, especially answer the previous pleading of the other party by denying facts therein stated or by alleging new facts;

The Collaborative International Dictionary of English v.0.48:

Plead \Plead\, v. t.
   1. To discuss, defend, and attempt to maintain by arguments
      or reasons presented to a tribunal or person having
      uthority to determine; to argue at the bar; as, to plead a
      cause before a court or jury.
      [1913 Webster]

            Every man should plead his own matter. --Sir T.
                                                  More.
      [1913 Webster]

   Note: In this sense, argue is more generally used by lawyers.
         [1913 Webster]

   2. To allege or cite in a legal plea or defense, or for
      repelling a demand in law; to answer to an indictment; as,
      to plead usury; to plead statute of limitations; to plead
      not guilty. --Kent.
      [1913 Webster]

   3. To allege or adduce in proof, support, or vendication; to
      offer in excuse; as, the law of nations may be pleaded in
      favor of the rights of ambassadors. --Spenser.
      [1913 Webster]

            I will neither plead my age nor sickness, in excuse
            of faults.                            --Dryden.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plead \Plead\, v. t. [imp. & p. p. Pleaded (colloq. Pleador
   Pled); p. pr. & vb. n. Pleading.] [OE. pleden, plaiden,
   OF. plaidier, F. plaider, fr. LL. placitare, fr. placitum.
   See Plea.]
   1. To argue in support of a claim, or in defense against the
      claim of another; to urge reasons for or against a thing;
      to attempt to persuade one by argument or supplication; to
      speak by way of persuasion; as, to plead for the life of a
      criminal; to plead with a judge or with a father.
      [1913 Webster]

            O that one might plead for a man with God, as a man
            pleadeth for his neighbor!            --Job xvi. 21.
      [1913 Webster]

   2. (Law) To present an answer, by allegation of fact, to the
      declaration of a plaintiff; to deny the plaintiff's
      declaration and demand, or to allege facts which show that
      ought not to recover in the suit; in a less strict sense,
      to make an allegation of fact in a cause; to carry on the
      allegations of the respective parties in a cause; to carry
      on a suit or plea. --Blackstone. Burrill. Stephen.
      [1913 Webster]

   3. To contend; to struggle. [Obs.] --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plead
    v 1: appeal or request earnestly; "I pleaded with him to stop"
    2: offer as an excuse or plea; "She was pleading insanity"
    3: enter a plea, as in courts of law; "She pleaded not guilty"
    4: make an allegation in an action or other legal proceeding,
       especially answer the previous pleading of the other party by
       denying facts therein stated or by alleging new facts

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "plead":
   adduce, adjure, advance, advocate, affirm, allege, appeal,
   appeal to, apply to, argue, argufy, array, ask, ask for, assert,
   aver, avow, bandy words, beg, beseech, bicker, blandish, brace,
   bring forward, bring on, bring to bear, cajole, call for help,
   call on, call upon, cavil, choplogic, clamor for, coax,
   conduct pleadings, conjure, contend, contest, crave, cross swords,
   cry for, cry on, cry to, cut and thrust, declare, demand, deploy,
   discept, dispute, entreat, exhort, give and take, hassle,
   have it out, high-pressure, impetrate, implead, implore, importune,
   imprecate, insist, insist upon, invoke, jawbone, join issue,
   kneel to, lobby, lock horns, logomachize, maintain, make a plea,
   marshal, moot, nag, obtest, offer, petition, pettifog, plead for,
   plead with, polemicize, polemize, pray, present, press, pressure,
   produce, push, put forward, quibble, rally, recommend, request,
   rest, run to, say, seek, soft-soap, solicit, spar, supplicate,
   swear, sweet-talk, take sides, thrash out, try conclusions, urge,
   wheedle, work on, wrangle