A CAPTCHA (an initialism for "Completely Automated Public Turing test to tell Computers and Humans Apart", trademarked by Carnegie Mellon University) is a type of challenge-response test used in computing to determine whether or not the user is human. The term was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper of Carnegie Mellon University, and John Langford of IBM. A common type of CAPTCHA requires that the user type the letters of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen.
*en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
This somewhat more sophisticated CAPTCHA of "wikipedia" adds more distortions as well as highlights, shadows, and random line segments to thwart edge detection.
Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, a CAPTCHA is sometimes described as a reverse Turing test, although this term is ambiguous because it could also mean a Turing test in which the participants are both attempting to prove they are the computer.
