Biorhythm can refer to two different concepts. Scientifically biorhythm can be shorthand for biological rhythms which are the recurring cycles in an organism's physiology (such as sleeping and waking), as well as routine emotional, physical, or mental activity.
One important biorhythm is the Circadian rhythm - the daily (diurnal) rhythms of day and night which occurs on a roughly 24-25 hour schedule. Biorhythms can also refer to predictions of behaviors and experiences using physical, emotional, and intellectual 'cycles.' In the 19th century, this theory was popularized by Wilhelm Fleiss and used biorhythm cycles as a predictive agent for things such as health, well-being, and personal success. This theory, considered a branch of pseudoscience, had its most recent reappearance in the 1970's, but has never been scientifically proven to be valid.