A clicker is any device that makes a clicking sound, usually when deliberately activated by its user.

They usually consist of a piece of thin metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 or plastic held in a casing so that the metal is slightly torqued; depressing one end of the metal causes it to pop out of alignment and releasing it causes it to pop back into alignment, each time making a sharp click.

With some clickers, the user depresses the metal directly with thumb or finger; with others, a button extends above the surface of the casing so that depressing the button makes the metal click.

Use in animal training

Clickers were first used by marine mammal trainer Karen Pryor as a way of communicating with their animals. Dolphins and whales communicate underwater through a series of clicks and whistles known as echolocation

Animal echolocation

Echolocation, also called biosonar, is the biological sonar used by several kinds of animals.Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects...

, and the clicker allowed a trainer to produce signals they were more likely to understand.

Nowadays, clickers are used to train all kinds of animals, most commonly dogs. When associated with a treat, a click allows the owner to mark the precise moment the desired behavior is executed.

Other use

• In contemporary society, clickers are often used by pet owners (particularly dog
  Dog
  The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
  
  s and Gambian pouched rats) as a behavioral tool
  Clicker training
  Clicker training is an operant conditioning method for training an animal using a clicker, or small mechanical noisemaker, as a marker for behavior. The method uses positive reinforcement - it is reward based...
  
  .
• Clickers are used to provide audible feedback for human students learning using a method called TAGteach.
• In World War II
  World War II
  World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
  
   Clickers were used by Allied paratroopers preceding and during Operation Overlord
  Operation Overlord
  Operation Overlord was the code name for the Battle of Normandy, the operation that launched the invasion of German-occupied western Europe during World War II by Allied forces. The operation commenced on 6 June 1944 with the Normandy landings...
  
   as a way of covertly identifying friend from foe. A soldier would click once and if two clicks were received in return from an unidentifiable soldier then his identification was confirmed.
• Clickers are also used as a handheld counting device, sometimes digital but more commonly mechanical, used to keep a count of the numbers of people entering a venue. It is often used by nightclub doorstaff to make sure fire limits are not exceeded.
• A Clicker is a device used on recurve bow
  Recurve bow
  In archery, the shape of the bow is usually taken to be the view from the side. It is the product of the complex relationship of material stresses, designed by a bowyer...
  
  s to signal to the archer that correct draw length has been achieved, thus aiding consistency.
• Some board games designed after game shows come with clickers that are meant to emulate the buzzers common on such shows.
• Clicker is slang for a television remote control, so named because of the clicking sound produced by the device and by the television when it changed channels.