Definition: nocturnal

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Source: WordNet (r) 1.7 

nocturnal
     adj 1: (biology) belonging to or active during the night;
            "nocturnal animals are active at night"; "nocturnal
            plants have flowers that open at night and close by
            day" [ant: diurnal]
     2: of or relating to or occurring in the night; "nocturnal
        darkness"
     3: of or during or relating to the night; "a nocturnal
        journey"; "nocturnal stillness"; "nocturnal predators"

Source: Webster's Revised Unabridged Dictionary (1913) 

Nocturnal \Noc*tur"nal\, n.
   An instrument formerly used for taking the altitude of the
   stars, etc., at sea. --I. Watts.
Nocturnal \Noc*tur"nal\, a. [L. nocturnalis, nocturnus, fr. nox,
   noctis, night. See Night, and cf. Nocturn.]
   1. Of, pertaining to, done or occuring in, the night; as,
      nocturnal darkness, cries, expedition, etc.; -- opposed to
      diurnal. --Dryden.

   2. Having a habit of seeking food or moving about at night;
      as, nocturnal birds and insects.