Frequency

Frequency refers to how often an event occurs. High frequency events, like waves, are measured in Hertz. Lower frequency events, like comets, are often referred to by terms like "twice a century" or even how many years until an event reoccurs. Events like comets happen with predictable regularity, but a "once in 100-year flood" does not refer to frequency, rather statistical probability (every year there is a 1/100 chance that the flood would occur).

To learn more about frequency from a Physics perspective, check out Hyperphysics.