Coun"ter*part` (koun"t?r-p?rt`), n. 1. A part corresponding to another part; anything which answers, or corresponds, to another; a copy; a duplicate; a facsimile.

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.

2. (Law) One of two corresponding copies of an instrument; a duplicate.

3. A person who closely resembles another.

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.

O counterpart
Of our soft sex, well are you made our lords.
Dryden.

• something that resembles something else, or has corresponding functions or characteristics
  
• either of two parts that fit together, or complement one another