CAPTCHA
Definition:
CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart and is a spam prevention tool used on Internet web sites. CAPTCHA commonly appears on blog sites as a series of random letters or numbers, sometimes with a squiggly line through them, that a human must replicate before submitting a comment on the blog site.
This extra step of human intervention prevents malicious robots or scripts from automatically submitting spam comments on a blog without the author's permission. In theory, CAPTCHA numbers and letters are unreadable by computers meaning a comment can only be submitted if a human verifies the numbers and letters.