The Collaborative International Dictionary of English v.0.48:

Sestet \Ses*tet"\, n. [It. sestetto, fr. sesto sixth, L. sextus,
   fr. sex six.]
   1. (Mus.) A piece of music composed for six voices or six
      instruments; a sextet; -- called also sestuor. [Written
      also sestett, sestette.]
      [1913 Webster]

   2. (Poet.) The last six lines of a sonnet.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sestet
    n 1: 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]
    2: six performers or singers who perform together [syn:
       sextet, sextette, sestet]
    3: a set of six similar things considered as a unit [syn:
       sextet, sextette, sestet]
    4: a musical composition written for six performers [syn:
       sextet, sextette, sestet]
    5: a rhythmic group of six lines of verse