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1.
Acronym: Completely Automated Public Turing test to Tell Computers and Humans Apart.

Tests that are easy for a human to pass, but difficult for a computer (thus preventing things like automated signups).
Ever see those registration forms that show a distorted picture and asks you to identify the word in it? That's a CAPTCHA.
by Spoom June 10, 2004
 
2.
that annoying thing with letters and number (e.g. A7FF9YTP) that sucks a lot when you do it wrong cause you have go back and do another one.
i friggin hate captchas
by rottonknickers May 03, 2008
 
3.
An image with random letters and/or numbers used to prevent spammers from using computer programs to automatically register accounts and/or post/send unwanted advertisements.
This website doesn't even have a CAPTCHA. That's why there's so much spam.
by captcha April 16, 2010
 
4.
to shoot at in cold blood
I know I captcha, man, but that was a long time ago.
by The45er March 18, 2006