Reverberation

Reverberation is when sound waves continue to vibrate after the original source of the sound has stopped emitting sound. Reverberation can cause echoes.
Reverberation time is a period when the sound is gone in an enclosed area after the source of the sound has stopped. In the end of 19th century Wallace Clement Sabine experimented with reverberation time. He determined the relationship between the quality of acoustics, the size of the chamber, and the amount of absorption surfaces present.