of or relating to a day or each day; daily.
of or belonging to the daytime (opposed to nocturnal ).
Botany. showing a periodic alteration of condition with day and night, as certain flowers that open by day and close by night.
active by day, as certain birds and insects (opposed to nocturnal ).
Liturgy. a service book containing offices for the daily hours of prayer.
Archaic. a diary.
Archaic. a newspaper, especially a daily one.

1400–50; late Middle English < Latin diurnālis, equivalent to diurn(us) daily + -ālis -al1

diurnally, adverb
diurnalness, noun
transdiurnal, adjective
undiurnal, adjective
undiurnally, adverb

diurnal, nocturnal.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
diurnal (daɪˈɜːnəl)
1.  happening during the day or daily
2.  (of flowers) open during the day and closed at night
3.  Compare nocturnal (of animals) active during the day
4.  a service book containing all the canonical hours except matins
[C15: from Late Latin diurnālis, from Latin diurnus, from diēs day]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

late 14c., from L.L. diurnalis, from diurnum "day," from L. diurnus "daily," from dies "day" + -urnus, an adj. suffix denoting time (cf. hibernus "wintery"). Dies "day" is from PIE base *dyeu- (cf. Skt. diva "by day," Welsh diw, Bret. deiz "day;" Arm. tiw; Lith. diena; O.C.S. dini, Pol. dzien, Rus.
den), lit. "to shine" (cf. Gk. delos "clear;" L. deus, Skt. deva "god," lit. "shining one;" Avestan dava- "spirit, demon;" Lith. devas, O.N. tivar "gods;" O.E. Tig, gen. Tiwes, see Tuesday).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

diurnal di·ur·nal (dī-ûr'nəl)

  1. Having a 24-hour period or cycle; daily.

  2. Occurring or active during the daytime rather than at night.

di·ur'nal·ly adv.
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
diurnal   (dī-ûr'nəl)  Pronunciation Key 
    1. Occurring once in a 24-hour period; daily.

    2. Having a 24-hour cycle. The movement of stars and other celestial objects across the sky are diurnal.

  1. Most active during the daytime. Many animals, including the apes, are diurnal.

  2. Having leaves or flowers that open in daylight and close at night. The morning glory and crocus are diurnal. Compare nocturnal.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Charles's many books yield an abundance of light/sky imagery, both diurnal and nocturnal.
That diurnal migration is something that the research team here hopes to shed
  some light on.
Furthermore, he said, some insects are particularly sensitive to changes in
  diurnal variations.
This interplay is given its diurnal rhythm by environmental influences, social
  habits and behavior patterns.
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