What CAPTCHA stands for
2026-10-04 · Captcha basics
CAPTCHA is a name from 2000 for a test a computer can grade, meant to tell a person from a program.
CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. The words were chosen so the initials spell a word people can say. Researchers at Carnegie Mellon University, including Luis von Ahn, Manuel Blum, Nicholas Hopper, and John Langford, introduced the name in 2000.
The "Turing" part points at Alan Turing's 1950 question about whether a machine can imitate a person in a conversation. A captcha turns that around. The judge is the website, and the subject is whoever is submitting the form. The site asks for something that, at the time the test was designed, a person could do and a script could not.
"Completely automated" means the site makes a new test and checks the answer without a human moderator. "Public" means the method can be published. The security was supposed to sit in the hardness of the puzzle, even when everyone knows how the puzzle is built.
A modern captcha is still that idea, applied to whatever is hard for programs in that year. Warped letters were the first widely used form. Checkboxes, photo grids, and risk scores came later, as programs learned to read text. The types of captchas and the history cover that change.
Captcha OCR reads a still image of characters or a short sum. It does not sit in the browser and it does not answer a checkbox widget. For the picture itself, start with what a captcha is.
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