Definition: This is short for "Completely Automated Public Turing test to tell Computers and Humans Apart" and the term is trademarked by Carnegie Mellon University. Many websites use a Captcha to prevent accounts being created automatically by web-bots- this happens particularly on free email websites where spammers continually require new accounts. Typically a Captcha has text in different fonts, colors and angles that make it difficult for a computer program to read but hopefully not for a human.
It's a bit of an arms race as some of the earlier captchas have been cracked by software.
Glossary:
A B C D E F G H I J K L M N O P Q R S T U V W X Y ZAlso Known As: Turning Test
