Source : Webster's Revised Unabridged Dictionary (1913)

Concerto \Con*cer"to\ (?; It. ?), n.; pl. {Concertos}. [It. See
   {Concert}, n.] (Mus.)
   A composition (usually in symphonic form with three
   movements) in which one instrument (or two or three) stands
   out in bold relief against the orchestra, or accompaniment,
   so as to display its qualities or the performer's skill.

Source : WordNet®

concerto
     n : a composition for orchestra and a soloist
     [also: {concerti} (pl)]