The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster] Concert of Europe

WordNet (r) 3.0 (2006):

concerto
    n 1: a composition for orchestra and a soloist