Source : Webster's Revised Unabridged Dictionary (1913)

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["a]hren;
   akin to OFries. wera. Cf. {Guarantee}.]
   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:
      (a) A writing which authorizes a person to receive money
          or other thing.
      (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.
      (c) (Mil. & Nav.) An official certificate of appointment
          issued to an officer of lower rank than a commissioned
          officer. See {Warrant officer}, below.

   2. That which vouches or insures for anything; guaranty;
      security.

            I give thee warrant of thy place.     --Shak.

            His worth is warrant for his welcome hither. --Shak.

   3. That which attests or proves; a voucher.

   4. Right; legality; allowance. [Obs.] --Shak.

   {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.

Source : WordNet®

warrant officer
     n : holds rank by virtue of a warrant