of or pertaining to the night (opposed to diurnal ).
done, occurring, or coming at night: nocturnal visit.
active at night (opposed to diurnal ): nocturnal animals.
opening by night and closing by day, as certain flowers (opposed to diurnal ).
Archaic. an astrolabe for telling time at night or for determining latitude by the position of certain stars in reference to Polaris.

1475–85; < Late Latin nocturnālis. See nocturn, -al1

nocturnality, noun
nocturnally, adverb
nonnocturnal, adjective
nonnocturnally, adverb
seminocturnal, adjective
unnocturnal, adjective
unnocturnally, adverb

diurnal, nocturnal.

2. nighttime. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
nocturnal (nɒkˈtɜːnəl)
1.  of, used during, occurring in, or relating to the night
2.  (of animals) active at night
3.  (of plants) having flowers that open at night and close by day
[C15: from Late Latin nocturnālis, from Latin nox night]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1485, from M.Fr. nocturnal, from L.L. nocturnalis, from L. nocturnus "belonging to the night," from nox (gen. noctis) "night," cognate with O.E. neaht (see night) + -urnus, suffix forming adjectives of time. Nocturnal emission "involuntary ejaculation during sleep" first recorded 1821.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

nocturnal noc·tur·nal (nŏk-tûr'nəl)

  1. Of, relating to, or occurring in the night.

  2. Most active at night.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
nocturnal   (nŏk-tûr'nəl)  Pronunciation Key 
  1. Occurring at night.

  2. Most active at night. Many animals, such as owls and bats, are nocturnal.

  3. Having flowers that open during the night and close at daylight. Nocturnal plants are often pollinated by moths. Compare diurnal.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
The diurnal ones prefer the daylight hours, while nocturnal species haunt the
The sundial's nocturnal counterpart, the water clock, was designed to measure
  temporal hours at night.
The illustrations are a blend of realism and fantasy, and set the mood for this
  nocturnal caper.
If only I were still a nocturnal creature.
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