ADJECTIVE (2)

1. belonging to or active during the night;
- Example: "nocturnal animals are active at night"
- Example: "nocturnal plants have flowers that open at night and close by day"

2. of or relating to or occurring in the night;
- Example: "nocturnal darkness"

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Having a habit of seeking food or moving about at night;
      as, nocturnal birds and insects; raccoons are nocturnal.;
      -- of animals.
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Nocturnal \Noc*tur"nal\, n.
   An instrument formerly used for taking the altitude of the
   stars, etc., at sea. --I. Watts.
   [1913 Webster]