The Collaborative International Dictionary of English v.0.48:

Terzetto \Ter*zet"to\, n. [It., dim. of terzo the third, L.
   tertius. See Tierce.] (Mus.)
   A composition in three voice parts; a vocal (rarely an
   instrumental) trio.
   [1913 Webster] Tesla coil

WordNet (r) 3.0 (2006):

terzetto
    n 1: the cardinal number that is the sum of one and one and one
         [syn: three, 3, III, trio, threesome, tierce,
         leash, troika, triad, trine, trinity, ternary,
         ternion, triplet, tercet, terzetto, trey, deuce-
         ace]