An Etymological Dictionary of Astronomy and Astrophysics
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Being, having, characterized by, or occurring in approximately 24-hour periods or cycles, as of biological activity or function (Merriam-Webster.com).

From L. circa "about," → circum-, + diem, accusative singular of dies "day," → diurnal.

Any of several physical, mental and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism's environment. Circadian rhythms are found in most living things, including animals, plants and many tiny microbes.

→ circadian; → rhythm.