Derived from Latin circa diem, meaning about a day, about daily. (circa = about, approximately, around; dies = day.)
An adjective describing things or phenomena that happen/occur in approximately 24-hour periods or cycles, i.e., daily (more or less).
For example, circadian rhythm refers to our biological clock which, when left alone, appears to tick in 25-hour cycles, but seems to adjust to a 24-hour cycle when we perceive the change of the light between the day and the night.