A CAPTCHA is a type of challenge-response test that is used in computing to determine whether the user is human or not.
The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper and John Langford. The most common type of CAPTCHA was first invented in 1997 by two parallel groups: (1) Mark D. Lillibridge, Martin Abadi, Krishna Bharat and Andrei Z. Broder; and (2) were Reshef, Gili Raanan and Eilon Solan. This form of CAPTCHA requires the user to write the letters of a distorted image, sometimes with the addition of a darkened sequence of letters or digits that appears on the screen. 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.
This user identification procedure has received much criticism, especially from people with disabilities, but also from other people who feel that their daily work is slowed by distorted words that are difficult to read. It takes the average person approximately 10 seconds to solve a typical CAPTCHA.