The Collaborative International Dictionary of English v.0.48:

Diurnal \Di*ur"nal\, n. [Cf. F. diurnal a prayerbook. See
   Diurnal, a.]
   1. A daybook; a journal. [Obs.] --Tatler.
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   2. (R. C. Ch.) A small volume containing the daily service
      for the "little hours," viz., prime, tierce, sext, nones,
      vespers, and compline.
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   3. (Zool.) A diurnal bird or insect.
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The Collaborative International Dictionary of English v.0.48:

Diurnal \Di*ur"nal\, a. [L. diurnalis, fr. dies day. See
   Deity, and cf. Journal.]
   1. Relating to the daytime; belonging to the period of
      daylight, distinguished from the night; -- opposed to
      nocturnal; as, diurnal heat; diurnal hours.
      [1913 Webster]

   2. Daily; recurring every day; performed in a day; going
      through its changes in a day; constituting the measure of
      a day; as, a diurnal fever; a diurnal task; diurnal
      aberration, or diurnal parallax; the diurnal revolution of
      the earth.
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            Ere twice the horses of the sun shall bring
            Their fiery torcher his diurnal ring. --Shak.
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   3. (Bot.) Opening during the day, and closing at night; --
      said of flowers or leaves.
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   4. (Zool.) Active by day; -- applied especially to the eagles
      and hawks among raptorial birds, and to butterflies
      (Diurna) among insects.
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   Diurnal aberration (Anat.), the aberration of light arising
      from the effect of the earth's rotation upon the apparent
      direction of motion of light.

   Diurnal arc, the arc described by the sun during the
      daytime or while above the horizon; hence, the arc
      described by the moon or a star from rising to setting.

   Diurnal circle, the apparent circle described by a
      celestial body in consequence of the earth's rotation.

   Diurnal motion of the earth, the motion of the earth upon
      its axis which is described in twenty-four hours.

   Diurnal motion of a heavenly body, that apparent motion of
      the heavenly body which is due to the earth's diurnal
      motion.

   Diurnal parallax. See under Parallax.

   Diurnal revolution of a planet, the motion of the planet
      upon its own axis which constitutes one complete
      revolution.

   Syn: See Daily.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diurnal
    adj 1: of or belonging to or active during the day; "diurnal
           animals are active during the day"; "diurnal flowers are
           open during the day and closed at night"; "diurnal and
           nocturnal offices" [ant: nocturnal]
    2: having a daily cycle or occurring every day; "diurnal
       rotation of the heavens"

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "diurnal":
   annual, biannual, biennial, bimonthly, biweekly, catamenial,
   centenary, centennial, circadian, daily, daytime, decennial,
   everyday, fortnightly, hebdomadal, hourly, menstrual, momentary,
   momently, monthly, quarterly, quotidian, regular, secular,
   semestral, semiannual, semimonthly, semiweekly, semiyearly,
   tertian, triennial, weekly, yearly