[sur-key-dee-uh n, -kad-ee-, sur-kuh-dee-uh n]


noting or pertaining to rhythmic biological cycles recurring at approximately 24-hour intervals.

Origin of circadian

1955–60; < Latin circā about + di(ēs) day + -an Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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Contemporary Examples of circadian

British Dictionary definitions for circadian



of or relating to biological processes that occur regularly at about 24-hour intervals, even in the absence of periodicity in the environmentSee also biological clock

Word Origin for circadian

C20: from Latin circa about + diēs day
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for circadian

coined 1959 from Latin circa "about" (see circa) + diem, accusative singular of dies "day" (see diurnal). The original use is in circadian rhythm.

Online Etymology Dictionary, © 2010 Douglas Harper

Medicine definitions for circadian


[sər-kādē-ən, -kădē-, sûr′kə-dīən, -dē-]


Relating to biological variations or rhythms with a cycle of about 24 hours.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.