The noun captcha, or Captcha, CAPTCHA, denotes any of various authentication systems devised to enable a computer to distinguish human from computer input, typically in order to thwart spam or to prevent automated misuse of a web site. The word also denotes a manifestation of such a system, especially a string of distorted letters that a user must read and key in correctly to proceed.

This word is an acronym from the initial letters of completely automatic public Turing test to tell computers and humans apart, with punning allusion to capture and gotcha.

The earliest instance that I have found is from Robot solves Internet robot problem, by Byron Spice, published in the Pittsburgh Post-Gazette (Pittsburgh, Pennsylvania) of Sunday 21^st October 2001 (Manuel Blum (born 1938) is a professor of computer science at Carnegie Mellon University, Pittsburgh, Pennsylvania):

Manuel Blum was having trouble with his hand-held computer; so he turned to an Internet chat room for advice on how to recover a lost file. A seemingly helpful reply directed him to a Web site that would supposedly solve his problem.
But what he found there was junk—a vendor trying to sell him something.
“I had gotten stung by a ’bot,” Blum realized. Robots, or automated programs, now routinely scan through chat rooms and, masquerading as people, send messages designed to lure computer users to certain vendor sites.
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.
The Captcha program, http://www.captcha.net, is an example of a problem that had plagued Yahoo and other Internet service providers, but could easily be solved by selecting the right algorithm, or problem-solving method. Matching these real world problems with solutions already devised by theoretical computer scientists is the goal of a newly funded program at Carnegie Mellon called Aladdin, www-2.cs.cmu.edu/~aladdin/.
“If you’re in academia, you’re always looking for interesting problems,” said Udi Manber, Yahoo’s chief scientist and a former computer science professor at the University of Arizona. “If you’re in industry, like me, you’ve got too many interesting problems.”
Manber visited Carnegie Mellon about a year ago to discuss problems that faced Yahoo; Captcha was one of the projects that developed from that visit.
Manber acknowledges that computers may eventually solve the Captcha problem and that the test will need to be toughened, but Blum actually looks forward to the day that computers defeat the test.
If artificial intelligence programs can learn to read distorted characters, he said, they would also be able to read words off of almost any sort of document, including the odd-sized characters often used in print advertisements. That in turn would mean that virtually any document in the Library of Congress could be scanned and made easily available to computer users. And that would be a bigger boon to society than a Captcha test.
“I am confident that computers someday are going to blow us out of the water in terms of intelligence,” Blum said. “I’d like to be around when that happens.”

The term Turing test, or Turing’s test, denotes a test of a computer’s ability to exhibit intelligence, requiring that a human being should be unable to distinguish the computer from another human being using only replies to questions that are put to both.

The use of such a test was proposed by the English mathematician, wartime code-breaker and pioneer of computer science Alan Mathison Turing (1912-54) in Computing Machinery and Intelligence, published in the magazine Mind of October 1950. He called it the imitation game.