The Collaborative International Dictionary of English v.0.48:

Sextet \Sex*tet"\, Sextetto \Sex*tet"to\, n. (Mus.)
   See Sestet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sextet
    n 1: a musical composition written for six performers [syn:
         sextet, sextette, sestet]
    2: the cardinal number that is the sum of five and one [syn:
       six, 6, VI, sixer, sise, Captain Hicks, half a
       dozen, sextet, sestet, sextuplet, hexad]
    3: six performers or singers who perform together [syn:
       sextet, sextette, sestet]
    4: a set of six similar things considered as a unit [syn:
       sextet, sextette, sestet]
    5: six people considered as a unit [syn: sextet, sextette,
       sixsome]

Moby Thesaurus II by Grady Ward, 1.0:

168 Moby Thesaurus words for "sextet":
   German band, Hexateuch, Jewish star, Magen David, Philharmonic,
   Spenserian stanza, antistrophe, band, big band, bipartisanship,
   book, brass, brass band, brass choir, brass quintet, brass section,
   brasses, burden, callithumpian band, canto, chamber orchestra,
   chorus, 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, couplet, desks, distich,
   dixieland band, duet, duettino, duo, duumvirate, ecumenicalism,
   ecumenicism, ecumenism, ensemble, ensemble music, ensemble singing,
   envoi, epode, esprit, esprit de corps, estoile, fellow feeling,
   fellowship, gamelan orchestra, group, half a dozen, harmony,
   heptastich, hexachord, hexad, hexagon, hexagram, hexahedron,
   hexameter, hexapod, hexapody, hexarchy, hexastich, hexastyle,
   jazz band, joining of forces, joint effort, joint operation,
   jug band, line, mass action, measure, military band, monostich,
   morale, mutual assistance, mutualism, mutuality, octastich, octave,
   octet, orchestra, ottava rima, part music, part singing, part song,
   pentastich, polyphonic music, pooling, pooling of resources,
   pulling together, quartet, quatrain, quintet, ragtime band,
   reciprocity, refrain, rhyme royal, rock-and-roll group, septet,
   septuor, sestet, sextuplet, sise, six, six-pointed star, sixer,
   skiffle band, solidarity, stanza, star of David, stave, steel band,
   strain, street band, string band, string choir, string orchestra,
   string quartet, strings, strophe, swing band, syllable, symbiosis,
   symphony, symphony orchestra, synergism, synergy, team spirit,
   teamwork, tercet, terza rima, terzet, terzetto, tetrastich, trio,
   triplet, tristich, triumvirate, troika, united action, verse,
   waits, woodwind, woodwind choir, woodwind quartet, woodwinds