noun, plural: circadian rhythms
An endogenously-driven biological rhythm based on a 24-hour cycle of biological processes
Biological rhythms pertain to the synchronized cyclic pattern demonstrated by an organism as its response to a particular stimulus. Biological clock that synchronizes with biological rhythm may be endogenous or exogenous. An endogenous type is one in which the internal biological clock is the one that controls it. An example of it is the body temperature cycle. An exogenous type is one that which involves an external cue (i.e. zeitgeber). An example is the sleep and wakefulness pattern.
A circadian rhythm entails different biological mechanisms resulting in an endogenous rhythmic pattern of 24 hours. This 24-hour rhythmic pattern is driven by a circadian clock. The term circadian came from the Latin term circa meaning around or approximately and diem meaning a day. Circadian rhythms are demonstrated by different groups of organisms. They occur in plants, animals, fungi, and cyanobacteria.1
A biological rhythm is considered as circadian when these three criteria are met: (1) an endogenous free-running (approximately) 24-hour period, (2) a rhythm that is entrainable, i.e. capable of phase reset by environmental cues and synchronization to the 24-h day, and (3) exhibiting temperature compensation.2
1 Edgar, R. S., Green, Edward, W., Zhao, Y., van Ooijen, G., Olmedo, M., Qin, X., Xu, Y., Pan, M., and Valekunja, U. K. (24 May 2012). "Peroxiredoxins are conserved markers of circadian rhythms". Nature. 485 (7399): 459–464.
2 Johnson, C. (2004). Chronobiology: Biological Timekeeping. Sunderland, Massachusetts, USA: Sinauer Associates, Inc. pp. 67–105.