CAPTCHA

• A CAPTCHA is a program that can generate and grade tests that humans can pass but current computer programs cannot. 
• It is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a person.
• The term CAPTCHA (for Completely Automated Public Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University