Definition of circadian
: being, having, characterized by, or occurring in approximately 24-hour periods or cycles (as of biological activity or function) <circadian rhythms in activity>
Did You Know?
Just over fifty years ago, no one talked about "circadian rhythms" - because "circadian" hadn't even been coined yet. In 1959, a scientist formed the word from the Latin words circa ("about") and "dies" ("day"), and it caught on quickly. "Circadian" appeared in periodicals throughout the sixties, and appeared in a Merriam-Webster dictionary before the decade was up. Most often, it's seen and heard in the term "circadian rhythm," which refers to the inherent cycle of about 24 hours that appears to control various biological processes, such as sleep, wakefulness, and digestive activity. If you want to impress your friends, you can also use the term "circadian dysrhythmia," a fancy synonym of "jet lag."
Origin and Etymology of circadian
Latin circa about + dies day + English 2-an — more at deity
First Known Use: 1959
CIRCADIAN Defined for English Language Learners
Definition of circadian for English Language Learners
: relating to the regular changes in a person or thing that happen in 24-hour periods
Medical Definition of circadian
Learn More about circadian
Britannica English: Translation of circadian for Arabic speakers
Seen and Heard
What made you want to look up circadian? Please tell us where you read or heard it (including the quote, if possible).