Called "CAPTCHA," it's essentially a test that's easy for a human to complete, but difficult or impossible for a computer to do.
a computer-generated squiggly-letter test that must be typed in by Internet/Web users to prove they are human
In order to send an email about a New York Times article, the site makes you type in a captcha.
by 2001; acronym for Completely Automatic Public Turing test to tell Computers and Humans Apart
A type of test used to determine whether a request to a web site comes from a human or a computer program, typically by asking the user to perform some kind of image recognition task such as reading distorted text. The term was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University) and John Langford (of IBM) as a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart". CAPTCHA aims to prevent software tools from performing actions which might degrade the service, such as registering user accounts or automating the playing of a game.