Periodicity's the quality of occurring at regular intervals or periods (in time or space) and can occur in different contexts: In timing devices: A clock marks time at periodic intervals. A metronome ticks at periodic intervals of time. In science: Periodicity in time's often specified by its frequency 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. In physics, period's th… (More on Periodicity)