What does CAPTCHA stand for?

Answer

It is a test to tell people from bots.

Context

The acronym was coined at Carnegie Mellon University around 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford. The impetus was a problem reported by Yahoo: automated programs were flooding its chat rooms and signing up for services in bulk. The CMU group proposed a test that a program could generate and grade but could not pass itself.

The 'Turing test' in the name is a loose reference to Alan Turing's imitation game, in which a machine tries to pass as a human. A CAPTCHA reverses the roles: a computer is the judge, and it presents a puzzle such as distorted letters that humans read easily and 2000-era software did not. It is therefore not a true Turing test, since there is no conversation.

The idea has an arms-race quality. Optical character recognition and later machine learning improved enough that text puzzles became solvable by software, so sites moved to image selection and behavioral signals. Von Ahn's reCAPTCHA, launched in 2007 and acquired by Google in 2009, used the same effort to help digitize scanned books by asking users to read words that OCR could not. The acronym's capitalization is deliberate and the letters stand for the phrase given in the answer.