The Collaborative International Dictionary of English v.0.48:

Quintet \Quin*tet"\, Quintette \Quin*tette"\, n. [It. quintetto,
   dim. of quinto the fifth, a fifth part, from L. quintus the
   fifth: cf. F. quintette. See Quint.] (Mus.)
   A composition for five voices or instruments; also, the set
   of five persons who sing or play five-part music.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quintet
    n 1: a musical composition for five performers [syn: quintet,
         quintette]
    2: 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]
    3: five performers or singers who perform together [syn:
       quintet, quintette]
    4: a set of five similar things considered as a unit [syn:
       quintet, quintette, quintuplet, quintuple]
    5: five people considered as a unit [syn: quintet,
       quintette, fivesome]