The Collaborative International Dictionary of English v.0.48:

Nocturn \Noc"turn\, 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]