An internal temporal rhythm, corresponding to an approximate 24-hour period, that regulates the biological and/or behavioral processes of living beings. Can also be influenced by external cues (known as Zeitgebers): daylight, for example. Some people think that a circadian rhythm is known as the biological clock, but though the are related, they are different things (please click the link below to learn more).
Example: Body temperature drops and melatonin secretion increases during the evening in humans; both are processes by which the body signals its need to rest and prepares itself for sleep.
Here is a like of Frequently Asked Questions about Circadian Rhythm: http://www.nigms.nih.gov/publications/factsheet_circadianrhythms.htm
Fournier, G. (2010). Circadian Rhythm. Psych Central. Retrieved on April 19, 2015, from http://psychcentral.com/encyclopedia/2008/circadian-rhythm/