1. [adj]  (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"
2. [adj]  of or during or relating to the night; "a nocturnal journey"; "nocturnal stillness"; "nocturnal predators"
3. [adj]  of or relating to or occurring in the night; "nocturnal darkness"

Nocturnal
Nocturnal

more details ...

1. \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.
   

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