counterpart

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One that closely resembles another.
2. n. One that has the same functions and characteristics as another; a corresponding person or thing: Their foreign minister is the counterpart of our secretary of state.
3. n. A copy or duplicate of a legal paper.
4. n. One of two parts that fit and complete each other.
5. n. One that serves as a complement.

Century Dictionary and Cyclopedia

1. n. A correspondent part; a part that answers to another, as the several parts or copies of an indenture corresponding to the original; a copy; a duplicate.
2. n. The complement, as a certificate of hiring given by a tenant to his landlord on receiving from him a certificate of letting, or a bought note given to the seller on receiving the sold note.
3. n. A person or thing exactly resembling another or corresponding to another in appearance, character, position, influence, and the like; a representative; a match; a fellow.
4. n. One of two parts which fit each other, as a cipher and its key, or a seal and its impression; hence, a thing that supplements another thing or completes it, or a person having qualities wanting in another, and such as compensate for the other's deficiencies.
5. n. In music, the part to be arranged or used in connection with another: as, the bass is the counterpart to the treble.

Wiktionary

1. n. Either of two parts that fit together, or complement one another.
2. n. law A duplicate of a legal document.
3. n. One which resembles another
4. n. One which has corresponding functions or characteristics.

GNU Webster's 1913

1. n. A part corresponding to another part; anything which answers, or corresponds, to another; a copy; a duplicate; a facsimile.
2. n. (Law) One of two corresponding copies of an instrument; a duplicate.
3. n. A person who closely resembles another.
4. n. 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.

WordNet 3.0

1. n. a duplicate copy
2. n. a person or thing having the same function or characteristics as another