The Collaborative International Dictionary of English v.0.48:

Septet \Sep*tet"\, Septette \Sep*tette"\, n. [From L. septem
   seven, like duet, from L. duo.]
   1. A set of seven persons or objects; as, a septet of
      singers.
      [1913 Webster]

   2. (Mus.) A musical composition for seven instruments or
      seven voices; -- called also septuor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

septet
    n 1: the cardinal number that is the sum of six and one [syn:
         seven, 7, VII, sevener, heptad, septet,
         septenary]
    2: seven performers or singers who perform together [syn:
       septet, septette]
    3: a set of seven similar things considered as a unit [syn:
       septet, septette]
    4: seven people considered as a unit [syn: septet, septette,
       sevensome]
    5: a musical composition written for seven performers [syn:
       septet, septette]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "septet":
   Heptateuch, Spenserian stanza, antistrophe, bipartisanship, book,
   burden, canto, chorus, coaction, coadjuvancy, coadministration,
   coagency, cochairmanship, codirectorship, collaboration,
   collaborativeness, collectivism, collusion, commensalism,
   common effort, common enterprise, communalism, communism,
   communitarianism, community, complicity, concert, concord,
   concordance, concurrence, cooperation, cooperativeness, couplet,
   distich, duet, duettino, duo, duumvirate, ecumenicalism,
   ecumenicism, ecumenism, ensemble music, ensemble singing, envoi,
   epode, esprit, esprit de corps, fellow feeling, fellowship,
   harmony, heptachord, heptad, heptagon, heptahedron, heptameter,
   heptarchy, heptastich, hexastich, joining of forces, joint effort,
   joint operation, line, mass action, measure, monostich, morale,
   mutual assistance, mutualism, mutuality, octastich, octave, octet,
   ottava rima, part music, part singing, part song, pentastich,
   polyphonic music, pooling, pooling of resources, pulling together,
   quartet, quatrain, quintet, reciprocity, refrain, rhyme royal,
   septennate, septuor, sestet, seven, sevener, sextet, solidarity,
   stanza, stave, strain, strophe, syllable, symbiosis, synergism,
   synergy, team spirit, teamwork, tercet, terza rima, terzet,
   terzetto, tetrastich, trio, triplet, tristich, triumvirate, troika,
   united action, verse, week