rhythm

the basic rhythmic unit in a piece of music; "the piece has a fast rhythm"; "the conductor set the beat"

recurring at regular intervals

an interval during which a recurring sequence of events occurs; "the never-ending cycle of the seasons"

the arrangement of spoken words alternating stressed and unstressed elements; "the rhythm of Frost's poetry"

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)