WordNet (r) 3.0 (2006):

mirror image
    n 1: a likeness in which left and right are reversed [syn:
         mirror image, reflection, reflexion]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "mirror image":
   antipode, antipodes, antipole, antithesis, antonym, contra,
   converse, counter, counterbalance, countercheck, counterpoint,
   counterpoise, counterpole, counterterm, foil, inverse, obverse,
   offset, opposite, opposite number, reverse, setoff, the contrary,
   the other side, vis-a-vis