What does CAPTCHA stand for in the context of online human-verification tests?

CAPTCHA stands for “Completely Automated Public Turing test to tell Computers and Humans Apart.”

The term describes an automated challenge designed to distinguish human users from computer programs. Early CAPTCHAs commonly showed distorted letters or numbers that users had to type, based on the assumption that people could recognize the characters more reliably than software.

The name refers to Alan Turing’s proposed test of machine intelligence, but a CAPTCHA has the opposite practical goal: it asks whether the participant is likely to be human. Luis von Ahn, Manuel Blum, Nicholas Hopper, and John Langford introduced the term in 2000. Modern systems such as reCAPTCHA may rely more on behavioral signals, image tasks, or background interaction than on distorted text. A CAPTCHA is not a general measure of intelligence; it is a security mechanism designed to reduce automated abuse such as spam and credential attacks.