WordNet (r) 3.0 (2006):

opposite number
    n 1: a person or thing having the same function or
         characteristics as another [syn: counterpart, opposite
         number, vis-a-vis]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "opposite number":
   antipode, antipodes, antipole, antithesis, antonym, coequal,
   compeer, complement, contra, converse, coordinate, counter,
   counterbalance, countercheck, counterpart, counterpoint,
   counterpoise, counterpole, counterterm, cousin, ditto, equal,
   equipollent, equivalent, fellow, foil, inverse, like, match, mate,
   obverse, offset, opposite, parallel, peer, reverse, rival, setoff,
   tally, the contrary, the other side, twin, vis-a-vis