CAPTCHA

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

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.