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]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "quintet":
   Pentateuch, Philharmonic, Phoebe, V, band, big band,
   bipartisanship, brass, brass band, brass choir, brass quintet,
   brass section, brasses, callithumpian band, chamber orchestra,
   cinque, coaction, coadjuvancy, coadministration, coagency,
   cochairmanship, codirectorship, collaboration, collaborativeness,
   collectivism, collusion, combo, commensalism, common effort,
   common enterprise, communalism, communism, communitarianism,
   community, complicity, concert, concert band, concord, concordance,
   concurrence, cooperation, cooperativeness, desks, dixieland band,
   duet, duettino, duo, duumvirate, ecumenicalism, ecumenicism,
   ecumenism, ensemble, ensemble music, ensemble singing, esprit,
   esprit de corps, fellow feeling, fellowship, fin, five, five bucks,
   five dollars, fiver, fivesome, gamelan orchestra, group, harmony,
   jazz band, joining of forces, joint effort, joint operation,
   jug band, mass action, military band, morale, mullet,
   mutual assistance, mutualism, mutuality, octet, orchestra,
   part music, part singing, part song, pentachord, pentad, pentagon,
   pentagram, pentahedron, pentameter, pentapody, pentarchy,
   pentastich, pentathlon, polyphonic music, pooling,
   pooling of resources, pulling together, quartet, quincunx, quint,
   quintuplet, ragtime band, reciprocity, rock-and-roll group, septet,
   septuor, sestet, sextet, skiffle band, solidarity, steel band,
   street band, string band, string choir, string orchestra,
   string quartet, strings, swing band, symbiosis, symphony,
   symphony orchestra, synergism, synergy, team spirit, teamwork,
   terzet, terzetto, trio, triumvirate, troika, united action, waits,
   woodwind, woodwind choir, woodwind quartet, woodwinds