Source : WordNet®

threesome
     n 1: the cardinal number that is the sum of one and one and one
          [syn: {three}, {3}, {III}, {trio}, {tierce}, {leash}, {troika},
           {triad}, {trine}, {trinity}, {ternary}, {ternion}, {triplet},
           {tercet}, {terzetto}, {trey}, {deuce-ace}]
     2: three people considered as a unit [syn: {trio}, {triad}, {trinity}]