What is the definition of a circadian rhythm?

Circadian rhythm is a term used in biology to describe the endogenous, self-sustaining oscillations in physiological, behavioral, and cognitive (including hormonal and metabolic) processes that occur over approximately a 24-hour period. These internal clocks, or biological rhythms, are closely synchronized to the local conditions and the surrounding environment, such as the daily light-dark cycle.

These naturally occurring cycles (known as circa = approximately, and diem = a day) are found in plants and animals, and help organisms anticipate and adapt to anticipated environmental changes. Essentially, circadian rhythms allow organisms to function in synchrony with their surroundings, aligning activities like feeding and sleeping to appropriate times of day.