Definition
Echolocation is a way of sensing in which an animal sends out its own sound or ultrasound, then detects the echo that bounces back off objects. From that echo it works out an object's distance, size, shape, and movement. Without seeing anything, it draws a 3D map of the world around it in its head, using only the ring of sound.
The secret to finding your way in a dark cave
Shout in a dark room with all the lights off. If the wall is close, the echo comes back in an instant. If the room is big, it comes back after a while. Bats and dolphins use this property of echoes with great precision. They send out ultrasound, a high-pitched sound that human ears can't hear, and then catch the moment it bounces back.
If you measure the time between the sound leaving and coming back, you can calculate the distance to an object. By comparing the differences in timing and loudness between the sound reaching the right ear and the left ear, they can also pinpoint which direction the object is in. That's why a bat can fly around with its eyes covered and still dodge a wire as thin as a human hair.
This method also works well in total darkness with no light at all, or deep in the sea where murky water blocks your view. It's an amazing survival strategy. Instead of visual information from the eyes, it uses hearing information from the ears to picture the surrounding space in 3D.
A little more precisely: it's not just distance
Echolocation isn't only a way to check where an obstacle is. When a bat hunts a single flying moth, it picks up even the speed of the moth's wingbeats and the direction it is flying.
When sound hits a moving object, the pitch of the returning sound changes slightly. It's like an ambulance siren, which sounds higher as it comes toward you and lower as it moves away. Animals read the change in frequency (the Doppler effect) of the returning echo to tell how fast their prey is escaping.
That's not all. The shape of the echo also changes depending on whether the surface it hit is smooth or rough. A bat's brain analyzes this fine texture of the sound waves. It can tell a hard twig from a soft moth wing.
Human technology that uses the same principle
Humans have built several technologies that work on the same principle as echolocation. They weren't copied from bats. They were developed separately. The best-known is SONAR (sound navigation and ranging), used on submarines and ships at sea. Light and radio waves don't travel far underwater, so these systems send out sound to find the seafloor terrain or other ships.
Radar, which sends radio waves through the air to track airplanes, and the ultrasound scanner a hospital uses to check a baby's health, are based on the same principle. They send out a wave, and a computer processes the signal that returns and shows it on a screen as a picture.
Some blind people even show an ability called human echolocation. They train with the echoes of tongue clicks and use them along with a cane to find the position of stairs or walls. It seems our ears and brains also hide the potential to turn echoes into information about space.
Common misconceptions
Bats are completely blind and depend only on echolocation.
All bats can see. Bats that echolocate mostly use this sense on nights with almost no light, or inside caves. Fruit-eating bats, the large bat group, mostly rely on sight and smell instead of echolocation.
Where you meet it
A bat in a dark cave precisely catching the wingbeats and position of an insect
A dolphin in muddy water making ultrasonic clicks to find a school of hidden fish
A submarine in the deep sea sending out sound waves to steer around undersea mountain ranges and reefs
In one sentence
It's the sense of sending out your own sound or ultrasound, then analyzing the echo that bounces back off objects to find their distance and shape.