abbrev. “Completely Automated Public Turing test to tell Computers and Humans Apart”; a challenge-response testing system; typically an image that contains a series of ambiguated characters that the reader must re-type in a given field.
Captcha was created mainly to combat the spam problem as a lot of “black hat” programmers write programs that automatically accesses some online services. These programs are made for various purposes, such as creating new email address, forum accounts or blog comments for the purpose of spamming.
Captcha codes typically feature a series of text, usually made up of words in the dictionary. To override OCR-enabled software, the text are warped and then displayed as an image file. The user is then required to re-type the text and return it to the server for verification. The user is allowed to use the site’s services only if the typed text matches the displayed text.
see also: Digital Marketing Tools
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Sitepoint (May 31, 2008)