Echo
Definition
An echo is a sound wave bouncing off a distant surface and returning to your ears — nature's way of measuring distance with time.
Technical definition
The distance to a reflector is d = v·t/2, where v is the wave speed and t is the round-trip time. In air at 20 °C, sound travels about 343 m/s, so a 1-second echo means a wall ~172 m away.
Example
Shout toward a cliff and hear the echo 2 seconds later: the cliff is 343×2/2 = 343 m away. Bats and sonar do the same calculation thousands of times per second.
Common confusion
An echo and reverberation are different. An echo is a distinct delayed repeat (reflector farther than ~17 m); reverberation is the smeared-out blending of many quick reflections in a small room.