WordNet (r) 3.0 (2006):

fivesome
    n 1: the cardinal number that is the sum of four and one [syn:
         five, 5, V, cinque, quint, quintet, fivesome,
         quintuplet, pentad, fin, Phoebe, Little Phoebe]
    2: five people considered as a unit [syn: quintet,
       quintette, fivesome]