Something that's circadian happens on a regular, daily basis. Your body's circadian rhythm is made up of processes that occur regularly every 24 hours.
You'll find the word circadian in biology class, or your anatomy and physiology textbook, because it describes a biological process in the human body, as well as in other organisms. Your cycle of sleep, for example, happens on a circadian schedule — approximately every 24 hours, you become tired enough to lie down and close your eyes, a built-in rhythm that's affected somewhat by light. Circadian comes from Latin roots, circa, "about," and diem, "day."