In 2000, Luis von Ahn, Manuel Blum, and Nicholas J. Hopper, affiliates
of Carnegie Mellon University, and IBM’s John Langford coined this term, which
stands for “Completely Automated Public Turing
test to tell Computers and Humans Apart.”
The test, administered by a computer, is different from the original Turing
test, which is typically administered by a human. It is a kind of
challenge-response test whose purpose is to ascertain whether a particular user
is a human. The test is frequently used to identify human users and block
computerized applications when signing up for some forms of Internet accounts.
An example of this use is to block spammers from automatically setting up email
accounts with free, public email services. The test involves the recognition of
a distorted image of letters, often with the inclusion of some obscure sequence
of numbers or letters.