Is a challenge-response system which is used to determine if the access to a web site is coming from a computer or a human being.

The term "CAPTCHA" was coined in 2000 by Luis von Ahn, Manuel Blum Nicholas J. Hopper (all of Carnegie Mellon University), and John Langford (then of IBM). It is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart". The term was attempted to be trademarked by Carnegie Mellon University, however the trademark application was abandoned on April 21st, 2008.

Unfortunately, the CAPTCHA tool was broken by companies desiring to exploit sites using the tools.  One of the most famous exploits of CAPTCHA was the compromise of ticketmaster selling tickets to Hannah Montanna concerts.

The circumvention of CAPTCHAs may violate the anti-circumvention clause of the Digital Millennium Copyright Act (DMCA) in the United States. In 2007, Ticketmaster sued software maker RMG Technologies for its product which circumvented the ticket seller's CAPTCHAs on the basis that it violates the anti-circumvention clause of the DMCA. In October 2007, an injunction was issued stating that Ticketmaster would likely succeed in making its case. In June 2008, Ticketmaster filed for Default Judgment against RMG. The Court granted Ticketmaster the Default and entered an $18.2M judgment in favor of Ticketmaster.