1.  diurnal(a = adj.all) - of or belonging to or active during the day; "diurnal and nocturnal offices"; "diurnal animals are active during the day"; "diurnal flowers are open during the day and closed at night"
Antonym: nocturnal

2.  diurnal(s = adj.all) - having a daily cycle or occurring every day; "diurnal rotation of the heavens"

diurnal, adj.
1 of or during the day; not nocturnal.
2 daily; of each day.
3 Astron. occupying one day.
4 Zool. (of animals) active in the daytime.
5 Bot. (of plants) open only during the day.

Derivative:
diurnally adv.

Etymology:
ME f. LL diurnalis f. L diurnus f. dies day

N  regularity of recurrence, periodicity, intermittence, beat, oscillation, pulse, pulsation, rhythm, alternation, alternateness, alternativeness, alternity, bout, round, revolution, rotation, turn, say, anniversary, jubilee, centenary, catamenia, courses, menses, menstrual flux, rota, cycle, period, stated time, routine, days of the week, Sunday, Monday, months of the year, January, feast, fast, Christmas, Easter, New Year's day, Allhallows, Allhallowmas, All Saints' Day, All Souls', All Souls' Day, Ash Wednesday, bicentennial, birthday, bissextile, Candlemas, Dewali, groundhog day, Halloween, Hallowmas, Lady day, leap year, Midsummer day, Muharram, woodchuck day, St. Swithin's day, natal day, yearbook, yuletide, punctuality, regularity, steadiness, periodic, periodical, serial, recurrent, cyclical, rhythmical, recurring, intermittent, remittent, alternate, every other, hourly, diurnal, daily, quotidian, tertian, weekly, hebdomadal, hebdomadary, biweekly, fortnightly, bimonthly, catamenial, monthly, menstrual, yearly, annual, biennial, triennial, centennial, secular, paschal, lenten, regular, steady, punctual, regular as clockwork, periodically, at regular intervals, at stated times, at fixed established, at established periods, punctually, de die in diem, from day to day, day by day, by turns, in turn, in rotation, alternately, every other day, off and on, ride and tie, round and round.