The Collaborative International Dictionary of English v.0.48:

Warrant \War"rant\, n. [OE. warant, OF. warant a warrant, a
   defender, protector, F. garant, originally a p. pr. pf German
   origin, fr. OHG. wer[=e]n to grant, warrant, G. gew[aum]hren;
   akin to OFries. wera. Cf. Guarantee.]
   [1913 Webster]
   1. That which warrants or authorizes; a commission giving
      authority, or justifying the doing of anything; an act,
      instrument, or obligation, by which one person authorizes
      another to do something which he has not otherwise a right
      to do; an act or instrument investing one with a right or
      authority, and thus securing him from loss or damage;
      commission; authority. Specifically:
      [1913 Webster]
      (a) A writing which authorizes a person to receive money
          or other thing.
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      (b) (Law) A precept issued by a magistrate authorizing an
          officer to make an arrest, a seizure, or a search, or
          do other acts incident to the administration of
          justice.
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      (c) (Mil. & Nav.) An official certificate of appointment
          issued to an officer of lower rank than a commissioned
          officer. See Warrant officer, below.
          [1913 Webster]

   2. That which vouches or insures for anything; guaranty;
      security.
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            I give thee warrant of thy place.     --Shak.
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            His worth is warrant for his welcome hither. --Shak.
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   3. That which attests or proves; a voucher.
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   4. Right; legality; allowance. [Obs.] --Shak.
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   Bench warrant. (Law) See in the Vocabulary.

   Dock warrant (Com.), a customhouse license or authority.

   General warrant. (Law) See under General.

   Land warrant. See under Land.

   Search warrant. (Law) See under Search, n.

   Warrant of attorney (Law), written authority given by one
      person to another empowering him to transact business for
      him; specifically, written authority given by a client to
      his attorney to appear for him in court, and to suffer
      judgment to pass against him by confession in favor of
      some specified person. --Bouvier.

   Warrant officer, a noncommissioned officer, as a sergeant,
      corporal, bandmaster, etc., in the army, or a
      quartermaster, gunner, boatswain, etc., in the navy.

   Warrant to sue and defend.
      (a) (O. Eng. Law) A special warrant from the crown,
          authorizing a party to appoint an attorney to sue or
          defend for him.
      (b) A special authority given by a party to his attorney
          to commence a suit, or to appear and defend a suit in
          his behalf. This warrant is now disused. --Burrill.
          [1913 Webster]

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

WARRANT OF ATTORNEY, practice. An instrument in writing, addressed to one or
more attorneys therein named, authorizing them generally to appear in any
court, or in some specified court, on behalf of the person giving it, and to
confess judgment in favor of some particular person therein named, in an
action of debt, and usually containing a stipulation not to bring any writ
of error, or file a bill in equity, so as to delay him.
     2. This general authority is usually qualified by reciting a bond which
commonly accompanies it, together with the condition annexed to it, or by a
written defeasance stating the terms upon which it was given, and
restraining the creditor from making immediate use of it. 31. In form it is
generally by deed; but it seems, it need not necessarily be so. 5 Taunt.
264.
     4. This instrument is given to the creditor as a security. Possessing
it, he may sign judgment and issue an execution, without its being necessary
to wait the termination. of an action. Vide 14 East, R. 576; 2 T. R. 100; 1
H. Bl. 75; 1 Str 20; 2 Bl. Rep. 1133; 2 Wils. 3; 1 Chit. Rep. 707.
     5. A warrant of attorney given to confess a judgment is not revocable,
and, notwithstanding a revocation, judgment may be entered upon it. 2 Ld.
Raym. 766, 850; 1 Salk. 87; 7 Mod. 93; 2 Esp, Rep. 563. The death of the
debtor is, however, generally speaking, a revocation. Co. Litt. 62 b; 1
Vent. 310. Vide Hall's Pr. 14, n.
     6. The virtue of a warrant of attorney is spent by the entry of one
judgment, and a second judgment entered on the same warrant is irregular. 1
Penna. R. 245; 6 S. & R. 296: 14 S. & R. 170; Addis. R. 267; 2 Browne's R.
321, 3 Wash. C. C. R. 558. Vide, generally, 18 Eng. Com. Law Rep. 94, 96,
179, 209; 1 Salk. 402; 3 Vin. Ab. 291; 1 Sell. Pr. 374; Com. Dig. Abatement,
E 1, 2; Id. Attorney, B 7, 8; 2 Archbold's Pr. 12; Bing. on Judgments, 38;
Grah. Pr. 618; l Crompt. Pr. 316; 1 Troub. & Haly's Pr. 96.
     7. A warrant of attorney differs from a cognovit, actionem. (q.v.) See
Metc. & Perk. Dig. Bond, IV.