GNU Collaborative International Dictionary of English

Found one definition 

  1.       

    Nocturn , n. [F. nocturne, fr. L. nocturnus. See Nocturnal, and cf. Nocturne.]
    1.  An office of devotion, or act of religious service, by night.
      1913 Webster
    2. One of the portions into which the Psalter was divided, each consisting of nine psalms, designed to be used at a night service.
      Hook.

      1913 Webster