(KAP.chuh) n. A computer-generated test that humans can pass but computer programs cannot. Also: Captcha, CAPTCHA.

Example Citation:
With his Ph.D. student Luis von Ahn and others Dr. [Manuel] Blum devised a collection of cognitive puzzles based on the challenging problems of artificial intelligence. The puzzles have the property that computers can generate and grade the tests even though they cannot pass them. The researchers decided to call their puzzles Captchas, an acronym for Completely Automated Public Turing Test to Tell Computers and Humans Apart (on the Web at

One puzzle, called Gimpy, consists of a display of seven distorted, overlapping words chosen at random from a dictionary of simple words. Solving the puzzle requires identifying three of the seven words and typing them into the box provided. The Carnegie Mellon group also created a simplified version of Gimpy — a single distorted word displayed against a complicated background. It is now part of Yahoo's registration process.

Another Captcha, called Sounds, consists of a distorted, computer-generated sound clip containing a word or sequence of numbers. To solve the puzzle, a user must listen to the clip and type the word or numbers into the box provided.
—Sara Robinson, "Human or Computer? Take This Test," The New York Times, December 10, 2002

Earliest Citation:
It's become a major problem on the Internet, as has the use of 'bots to register for e-mail addresses that are later used to send unwanted advertisements, or spam, to e-mail users.

Blum's research team at Carnegie Mellon University has come up with a solution to the problem, one that the Web portal Yahoo implemented last month. Now, when computer users try to register with Yahoo, they must pass a test to verify that they are human, not a robot.

The test is administered by a computer program.

"Here you have a computer program that creates a test, administers it and grades it, but can't pass its own test," Blum said.

It's what Blum calls a "captcha," a "gotcha"-inspired acronym that means Completely Automatic Public Turing test to tell Computers and Humans Apart.

The captcha is based on the fact that people can easily analyze images that flummox computers. For the Yahoo site, new registrants must read a common word that has been twisted or distorted and then type it into a box. It's easy for humans; impossible for computers.
—Byron Spice, "Robot solves Internet robot problem," Pittsburgh Post-Gazette, October 21, 2001

As the example citation says, the word captcha began life as an acronym for the phrase Completely Automated Public Turing test to tell Computers and Humans Apart. What's a Turing test? In 1950, the mathematician Alan Turing suggested that a computer could earn the label "intelligent" if it could fool a person into thinking he or she was communicating with another person instead of a machine. A captcha is mostly interested in the opposite scenario: proving that a Web form (or whatever) is being submitted by a human instead of a computer program. This twist on Turing's original idea is why some people call captchas reverse Turing tests.

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