In audio signal processing and acoustics, an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or in a room, by the walls. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound.

If so many reflections arrive at a listener that he cannot distinguish between them, the proper term is reverberation.

An echo can be explained as a wave that has been reflected by a discontinuity in the propagation medium, and returns with sufficient magnitude and delay to be perceived.

The intensity of an echo is frequently measured in dB relative to the directly transmitted wave.

Echoes may be desirable (as in radar) or undesirable (as in telephone systems).

In computing, an echo is the printing or display of characters (a) as they are entered from an input device, (b) as instructions are executed, or (c) as retransmitted characters received from a remote terminal.

In computer graphics, an echo is the immediate notification of the current values provided by an input device to the operator at the display console.

Some information from Federal Standard 1037C and from MIL-STD-188.

Echo is also: