Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

   2. (Poet.) The last six lines of a sonnet.