The Collaborative International Dictionary of English v.0.48:

Quartet \Quar*tet"\, Quartette \Quar*tette"\, n. [It. quartetto,
   dim. of quarto the fourth, a fourth part, fr. L. quartus the
   fourth. See Quart.]
   1. (Mus.)
      (a) A composition in four parts, each performed by a
          single voice or instrument.
      (b) The set of four person who perform a piece of music in
          four parts.
          [1913 Webster]

   2. (Poet.) A stanza of four lines.
      [1913 Webster]

WordNet (r) 3.0 (2006):

quartet
    n 1: the cardinal number that is the sum of three and one [syn:
         four, 4, IV, tetrad, quatern, quaternion,
         quaternary, quaternity, quartet, quadruplet,
         foursome, Little Joe]
    2: four performers or singers who perform together [syn:
       quartet, quartette]
    3: a set of four similar things considered as a unit [syn:
       quartet, quartette, quadruplet, quadruple]
    4: four people considered as a unit; "he joined a barbershop
       quartet"; "the foursome teed off before 9 a.m." [syn:
       quartet, quartette, foursome]
    5: a musical composition for four performers [syn: quartet,
       quartette]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "quartet":
   German band, Little Joe, Philharmonic, band, big band,
   bipartisanship, biquadrate, brass, brass band, brass choir,
   brass quintet, brass section, brasses, callithumpian band,
   chamber orchestra, 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, four, four-part diaphony, foursome, gamelan orchestra,
   group, harmony, jazz band, joining of forces, joint effort,
   joint operation, jug band, mass action, military band, morale,
   mutual assistance, mutualism, mutuality, octet, orchestra,
   part music, part singing, part song, polyphonic music, pooling,
   pooling of resources, pulling together, quad, quadrangle,
   quadrature, quadrennium, quadrilateral, quadrille, quadrinomial,
   quadruplet, quatern, quaternary, quaternion, quaternity,
   quatrefoil, quintet, ragtime band, reciprocity, rectangle,
   rock-and-roll group, septet, septuor, sestet, sextet, skiffle band,
   solidarity, square, square dance, squaring, 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, tetrad, tetragon, tetragram, tetragrammaton,
   tetrahedron, tetralogy, tetraphony, tetrapody, trio, triumvirate,
   troika, united action, waits, woodwind, woodwind choir,
   woodwind quartet, woodwinds