From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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]