Generally speaking, the number of occurrences of an event in a specific time interval.
The term frequency is commonly used to refer to the number of cycles per second a musical pitch, electrical signal, or other analog data repeats, commonly measured in hertz (which corresponds to one cycle per second). Since analog signals—such as sounds or electrical signals—are essentially waves, the frequency is a measure of how many crests (or troughs) at a given amplitude pass a given point per unit time. In terms of sound, for example, low frequencies correspond to bass sounds; high frequencies correspond to treble.
In computers, the term frequency, expressed in megahertz (MHz), or millions of cycles per second, is a measure of a CPU's clock rate.