Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

nocturne
     n : a pensive lyrical piece of music (especially for the piano)