Source : Webster's Revised Unabridged Dictionary (1913)

Foursome \Four"some\, a. [Four + 2d -some.]
   Consisting of four; requiring four participants. [Scot. or
   Golf]

Foursome \Four"some\, n. (Golf)
   A game between four players, with two on each side and each
   side playing but one ball, the partners striking alternately.
   It is called a

   {mixed foursome} when each side consists of a man and a
      woman.

Source : WordNet®

foursome
     n 1: four people considered as a unit; "he joined a barbershop
          quartet"; "the foursome teed off before 9 a.m." [syn: {quartet},
           {quartette}]
     2: the cardinal number that is the sum of three and one [syn: {four},
         {4}, {IV}, {tetrad}, {quatern}, {quaternion}, {quaternary},
         {quaternity}, {quartet}, {quadruplet}, {Little Joe}]