CAPTCHA

A CAPTCHA is a test that is used to separate humans and machines. CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." It is normally an image test or a simple mathematics problem which a human can read or solve, but a computer cannot. It is made to stop computer hackers from using a program to automatically set up hundreds of accounts, such as email accounts. It is named after mathematician Alan Turing's Turing test.[1]

Wikipedia's ReCAPTCHA test, used before making certain changes or creating accounts. The letters are completely random.

References
  1. "A word is born". Reader's Digest. 176 (1054). Australia: 165. February 2010.
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