Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

terzetto
     n : 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}, {trey}, {deuce-ace}]
     [also: {terzetti} (pl)]