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]