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The frequency is a measurement of repeated occurrences of an event.
In statistics, the frequency can be the count of events during an entire experiment. The unit for frequency is Hertz (named after German physicist Heinrich Rudolf Hertz), and is the inverse of seconds, the unit for time. This requires that the count of occurrences is divided by the time duration of the experiment or measurement.
In measuring the frequency of sound, electromagnetic waves (such as radio or light) or electrical signals, the frequency in Hertz is the number of full cycles (of a sine wave) per second. Some (old) radio receivers had the dial measured in megacycles per second (Mc/s) rather than megahertz.
Frequency standards are defined by atomic clocks, and public access exists (see link).