The Collaborative International Dictionary of English v.0.48:

Counterpart \Coun"ter*part`\ (koun"t[~e]r*p[aum]rt`), n.
   1. A part corresponding to another part; anything which
      answers, or corresponds, to another; a copy; a duplicate;
      a facsimile.
      [1913 Webster]

            In same things the laws of Normandy agreed with the
            laws of England, so that they seem to be, as it
            were, copies or counterparts one of another. --Sir
                                                  M. Hale.
      [1913 Webster]

   2. (Law) One of two corresponding copies of an instrument; a
      duplicate.
      [1913 Webster]

   3. A person who closely resembles another.
      [1913 Webster]

   4. A thing may be applied to another thing so as to fit
      perfectly, as a seal to its impression; hence, a thing
      which is adapted to another thing, or which supplements
      it; that which serves to complete or complement anything;
      hence, a person or thing having qualities lacking in
      another; an opposite.
      [1913 Webster]

            O counterpart
            Of our soft sex, well are you made our lords.
                                                  --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

counterpart
    n 1: a person or thing having the same function or
         characteristics as another [syn: counterpart, opposite
         number, vis-a-vis]
    2: a duplicate copy [syn: counterpart, similitude, twin]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "counterpart":
   ally, alter ego, analogon, analogue, associate, brother, clone,
   close copy, close match, coequal, cognate, companion, compeer,
   complement, congenator, congener, coordinate, correlate,
   correlative, correspondent, ditto, double, dupe, duplicate,
   duplication, each other, equal, equipollent, equivalent, facsimile,
   fellow, image, kindred spirit, like, likeness, match, mate, model,
   near duplicate, obverse, one another, opposite number, parallel,
   peer, pendant, picture, quadruplicate, reciprocal, reciprocatist,
   reciprocator, repetition, replica, replication, representation,
   reproduction, rival, second self, similitude, simulacrum, sister,
   soul mate, such, suchlike, tally, the like of, the likes of,
   triplicate, twin, vis-a-vis



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

COUNTERPART, contracts. Formerly each party to an indenture executed a
separate deed; that part which was executed by the grantor was called the
original, and the rest the counterparts. It is now usual for all the parties
to execute every part, and this makes them all originals. 2 Bl. Com. 296.
     2. In granting lots subject to a ground rent reserved to the grantor,
both parties execute the deeds, of which there are two copies; although both
are original, one of them is sometimes called the counterpart. Vide 12 Vin.
Ab. 104; Dane's Ab. Index, h.t.; 7 Com. Dig. 443; Merl. Repert. mots Double
Ecrit.