opposite number

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

• noun A person who holds a position in an organization or system corresponding to that of a person in another organization or system; a counterpart.

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

• noun idiomatic A person who holds a position in an organization that corresponds to that held by another person in an other organization; a counterpart

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

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