Frequency is the number of cycles completed per second. It is the reciprocal of period. Note: The unit representing cycles per second is hertz.
definition
Define Periodic Motion
Periodic motion, motion repeated in equal intervals of time. Periodic motion is performed, for example, by a rocking chair, a bouncing ball, a vibrating tuning fork, a swing in motion, the Earth in its orbit around the Sun, and a water wave. In each case the interval of time for a repetition, or cycle, of the motion is called a period, while the number of periods per unit time is called the frequency.