A CAPTCHA ( KAP-chə) is a type of challenge–response test used in computing to determine whether the user is human in order to deter bot attacks and spam.

The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford. It is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart." A historically common type of CAPTCHA (displayed as reCAPTCHA v1) was first invented in 1997 by two groups working in parallel. This form of CAPTCHA requires entering a sequence of letters or numbers in a distorted image. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, CAPTCHAs are sometimes described as reverse Turing tests.

Two widely used CAPTCHA services are Google's reCAPTCHA and the independent hCaptcha. It takes the average person approximately 10 seconds to solve a typical CAPTCHA.