An electronic circuit which delays the passage of a signal through it, such that any given signal that is applied to the input will appear at the output after a precise amount of time has elapsed. Delay lines may be digital or analog in nature.  Contemporary analog delays are solid-state and are based on bucket brigade devices. Prior to about 1990, tape echo units were commonly used for this purpose.

When the output of a delay line is mixed with the original signal, a variety of audio effects are produced, depending to a considerable extent on the delay time.  These can be divided into five broad categories: