CAPTCHA is standing for “Completely Automated Public Turing test to tell Computers and Humans Apart”. It is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a person. Because other computers are supposedly unable to solve the CAPTCHA, it is sometimes described as a reverse Turing test, because it is administered by a machine and targeted to a human.
A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen.
The term “CAPTCHA” was launched in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie Mellon University).
A very interesting contribution about CAPTCHA (in french) has been published by Nicolas Kerschenbaum, a security consultant of the french company Xmco Partners.