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]