A circadian rhythm /sɜrˈkeɪdiən/ is any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi, and cyanobacteria.The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diēs, meaning "day".