What is the difference between echo and reverberation?

Echo and reverberation refers to the effect of sounds reflected off solid objects, such as walls or ceilings in a theater or rocks in a valley. They are differentiated by the length of time between the initial sound and the reflected repetition.

If the distance is short, such as in a room or theater, the sound will be reflected back to the source in less than one-tenth of a second. This effect is reverberation. Because there is such a small delay in the sound repetition, sometimes only a few milliseconds, reverberation is often perceived by a listener as adding fullness to the original sound. When reflected sound travels a greater distance, such as a river valley, and takes more than one-tenth second to return, it is referred to as an echo. Echo does not add to the original sound as reverberation does, but is perceived as a distinct repetition of the sound, usually slightly fainter than the original.