Captcha is a system for checking that a human and not a machine is using a computer. Captcha is an abbreviation for 'completely automated public Turing test to tell computers and humans apart'.
We use captcha to screen for spam.
captcha in British English
(ˈkæptʃə)
noun
a test designed to determine whether a computer user is a person or a machine, eg by asking the user to decipher a distorted image
Word origin
C21: acronym for C(ompletely) A(utomated) P(ublic Turing Test to) T(ell) C(omputers and) H(umans) A(part)