Frequency is the number of occurrences of a repeating event per unit time. It describes the number of waves that pass a fixed place in a given amount of time. So if the time it takes for a wave to pass is is 1/2 second, the frequency is 2 per second. If it takes 1/100 of an hour, the frequency is 100 per hour. For example, an "A" note on a violin string vibrates at about 440 Hz (440 vibrations per second).
The SI unit of frequency is the hertz (abbreviated Hz), named after the German physicist Heinrich Hertz. A previous name for this unit was cycles per second (cps). The SI unit for period is the second. A traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or rpm. 60 rpm equals one hertz.
