Frequency is the number of complete wavelengths that pass a given point in a given time. In sound it determines pitch; in light it determines hue.

For auditory stimuli, frequency is measured in hertz (cycles per second). Low-frequency waves are heard as low pitches; high-frequency waves as high pitches. The audible range for young humans runs roughly from 20 Hz to 20,000 Hz, with greatest sensitivity in the middle frequencies that carry speech. The basilar membrane of the cochlea performs a frequency analysis: different regions respond maximally to different frequencies, providing the place code for pitch.

In vision, frequency refers to the electromagnetic waves of light. Higher frequencies (shorter wavelengths) appear blue or violet; lower frequencies appear red. The visible spectrum is a narrow band within the much wider electromagnetic spectrum.

Frequency is a physical property of the wave. Pitch and hue are the psychological experiences that the nervous system constructs from it.