: the number of individuals in a single class when objects are classified according to variations in a set of one or more specified attributes
: the number of repetitions of a periodic process in a unit of time
Number of waves that pass a fixed point per unit time; also, the number of cycles or vibrations undergone in unit time by a body in periodic motion. Frequency f is the reciprocal of the time T taken to complete one cycle (the period), or 1/T. The frequency with which earth rotates is once per 24 hours. Frequency is usually expressed in units called hertz (Hz). One hertz is equal to one cycle per second; one kilohertz (kHz) is 1,000 Hz, and one megahertz (MHz) is 1,000,000 Hz. The musical pitch A above middle C (the A string of a violin) has been widely standardized as 440 Hz.