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]