"These two opposing actions and reactions constitute the electro-magnetic oscillations, or compression-expansion sequences, which are present in every mass and which constitute the mechanistic or life principle of the universe. These sequences have a periodicity of preponderances which are first preponderantly generative and then preponderantly degenerative, which we know of as the phenomena of growth." [Russell to NYT - 1930 November 2]
Periodicity in time is often specified by its frequency, the inverse of the repetition period, which has the metric units of hertz (the number of periods per second).
Oscillations, waves and standing waves have crests at periodic intervals of space and/or time .(Wikipedia)
noun: the quality of recurring at intervals. Repeating, cyclical.