A CAPTCHA or Captcha is a type of challenge-response test used incomputing to ensure that the response is not generated by a computer. The process usually involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. Thus, it is sometimes described as a reverse Turing test, because it is administered by a machine and targeted to a human, in contrast to the standard Turing test that is typically administered by a human and targeted to a machine. A common type of CAPTCHA requires that the user type letters or digits from a distorted image that appears on the screen.
The term "CAPTCHA" (based upon the wordcapture) was coined in 2000 by Luis von Ahn,Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie Mellon University). It is acontrived acronym for "Completely AutomatedPublic Turing test to tell Computers and HumansApart." Carnegie Mellon University attempted to trademark the term, but the trademark application was abandoned on 21 April 2008.
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