counterpart

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that closely resembles another.
• noun One that has the same functions and characteristics as another; a corresponding person or thing.
• noun A duplicate of a legal document.
• noun One of two parts that fit and complete each other.
• noun One that serves as a complement.

from The Century Dictionary.

• noun 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.
• noun 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.
• noun 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.
• noun 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.
• noun In music, the part to be arranged or used in connection with another: as, the bass is the counterpart to the treble.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Either of two parts that fit together, or complement one another.
• noun law A duplicate of a legal document.
• noun One which resembles another
• noun One which has corresponding functions or characteristics.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a duplicate copy
• noun a person or thing having the same function or characteristics as another