Frequency refers to how often cyclical changes occur within a specified time unit. Frequency is measured in Hz, which is pronounced "hurts" and comes from Heinrich Rudolph Hertz, a German physicist (1857-1894) who studied electromagnetic radiation. Period refers to how long one complete cycle takes, and is the reciprocal of frequency. These terms apply to events or signals that recur repeatedly, or periodically.

For example, suppose that you get 12 paychecks in a year and they are equally spaced in time. Thus, the frequency of getting paid is 12 cycles per year, where cycle is hereby defined to be the time between pay periods. The period is how long one cycle lasts, and is 1/12 (one twelfth) of a year in this example.