cir·ca·di·an \(ˌ)sərˈkādēən, ¦sərkə¦dēən\adjective Etymology: Latin circa about + dies day + English -an — more at circa deity : being, having, characterized by, or occurring in approximately 24-hour periods or cycles (as of biological activity or function) < circadian oscillations > < circadian periodicity > < circadian rhythms in activity > < circadian leaf movements > • circadianlyadverb