1. a synonym of biological rhythm.

2. according to pseudoscientific belief, a regular biological cycle that can be charted like a sine wave and with which every individual is said to be programmed at birth. Usually this term is used to refer to any one of three basic cycles—physical, emotional, and intellectual—although other cycles (e.g., aesthetic, spiritual) and combinations of cycles have been proposed. It is maintained that these cycles continue unaltered until death and that good and bad days for various activities can be calculated accordingly. As with astrology, predictions made on this basis do not have a significantly different success rate from those made on a basis of pure chance.