The Collaborative International Dictionary of English v.0.48:

Nocturne \Noc*turne"\, n. [F. See Nocturn.] (Mus.)
   A night piece, or serenade. The name is now used for a
   certain graceful and expressive form of instrumental
   composition, as the nocturne for orchestra in Mendelsohn's
   "Midsummer-Night's Dream" music.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nocturne
    n 1: a pensive lyrical piece of music (especially for the piano)
         [syn: nocturne, notturno]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "nocturne":
   Nachtmusik, absolute music, adaptation, air varie, aleatory,
   aleatory music, arrangement, chamber music, chamber orchestra,
   composition, descant, electronic music, etude, exercise,
   harmonization, incidental music, instrumental music, invention,
   opus, orchestration, piece, production, program music, ricercar,
   score, sonata, sonatina, string orchestra, string quartet, study,
   theme and variations, trio, variation, work