pairing
     n 1: the act of pairing a male and female for reproductive
          purposes; "the casual couplings of adolescents"; "the
          mating of some species occurs only in the spring" [syn:
          {coupling}, {mating}, {conjugation}, {union}, {sexual
          union}]
     2: the act of grouping things or people in pairs
Source: WordNet® 2.0