Definition of "Rhythm" :

noun: natural family planning in which ovulation is assumed to occur 14 days before the onset of a period (the fertile period would be assumed to extend from day 10 through day 18 of her cycle)

noun: recurring at regular intervals

noun: the arrangement of spoken words alternating stressed and unstressed elements

"The rhythm of Frost's poetry."

noun: the basic rhythmic unit in a piece of music

"The piece has a fast rhythm."

noun: an interval during which a recurring sequence of events occurs