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.

circadian: fluctuating regularly on a time basis usually entrained to a metabolic, endocrine, neurochemical, light-dark, temperature, or seasonal cycle (see also diurnal, ultradian). There are very fast biorhythm cycles such as in the EEG and ECG, a 90-minute cycle seen in REM-NREM sleep or in waking attentive/alert-drowsy/inattentive swing, a twice daily body temperature peak and slump. a 24-hour ACTH cycle, a monthly menstrual cycle, and possibly longer seasonal cycles not yet thoroughly described.

Dictionary of Sexology Project: Main Index