WordNet (r) 3.0 (2006):

musical composition
    n 1: a musical work that has been created; "the composition is
         written in four movements" [syn: musical composition,
         opus, composition, piece, piece of music]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MUSICAL COMPOSITION. The act of congress of February 3, 1831, authorizes the
granting of a copyright for a musical composition. A question was formerly
agitated whether a composition published on a single sheet of paper, was to
be considered a book, and it was decided in the affirmative. 2 Campb. 28,
n.; 11 East, 244. See Copyright.