It is a method to check if you are really interacting with a human and not with a machine, also the image or audio that is used for the test. As an acronym it is taken from the English phrase Completely automated public Turing test to tell computers and humans apart, which is precisely based on the Turing test to confirm if we would be able to recognize when we are dialoguing with a person or with an artificial intelligence. For this case the usual thing is to present publicly the image of a distorted text, which can not be interpreted by the OCR scanner of an AI but by a person, so it is also called "Inverse Turing", since the original test referred to recognizing a machine that simulates being human. See recaptcha, Voight-Kampf.
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