Frequency is a measure of how often something occurs over a given period of time. INCIDENCE is the number of occurrences of a particular event within a given population or area, usually expressed as a RATE or proportion. OSCILLATION is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. RECURRENCE is the repetition of an event or phenomenon at regular intervals. PITCH is the perceived frequency of a sound, which is determined by the RATE of vibrations producing the sound. RATE is the speed at which something happens or changes.