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]