Source : Webster's Revised Unabridged Dictionary (1913)

Trio \Tri"o\, n. [It., fr. L. tres, tria, three: cf. F. trio,
   from the Italian. See {Three}.]
   1. Three, considered collectively; three in company or acting
      together; a set of three; three united.

            The trio were well accustomed to act together, and
            were linked to each other by ties of mutual
            interest.                             --Dickens.

   2. (Mus.)
      (a) A composition for three parts or three instruments.
      (b) The secondary, or episodical, movement of a minuet or
          scherzo, as in a sonata or symphony, or of a march, or
          of various dance forms; -- not limited to three parts
          or instruments.

Source : WordNet®

trio
     n 1: the cardinal number that is the sum of one and one and one
          [syn: {three}, {3}, {III}, {threesome}, {tierce}, {leash},
           {troika}, {triad}, {trine}, {trinity}, {ternary}, {ternion},
           {triplet}, {tercet}, {terzetto}, {trey}, {deuce-ace}]
     2: a musical composition for three performers
     3: three performers or singers who perform together
     4: a set of three similar things considered as a unit [syn: {triad},
         {triplet}]
     5: three people considered as a unit [syn: {threesome}, {triad},
         {trinity}]