An "echo" is a sound that is reflected off a surface and heard again. This phenomenon occurs when sound waves travel through the air, hit a solid object, and bounce back to the listener. Common surfaces that create echoes include mountains, buildings, and large walls. Echoes are often used in various fields, such as sonar technology, where sound waves help locate objects underwater. They also play a role in music, where musicians may use echo effects to enhance their performances. Understanding echoes can help in fields like acoustics and architecture, where sound quality is important.