CAPTCHA

A CAPTCHA is a test designed to tell a human apart from an automated program (a bot). They are those small challenges that appear when you submit a form or create an account: typing some distorted letters, selecting the images that contain a certain object, or simply ticking an "I’m not a robot" box. Their purpose is to protect websites from automated abuse, such as comment spam ✕ Spam is unwanted bulk messages, usually by email — often advertising, but sometimes a carrier for phishing or malware. More in the glossary → , mass creation of fake accounts or large-scale attempts to guess passwords. The name is an acronym for "Completely Automated Public Turing test ✕ The Turing test is a test proposed by Alan Turing to decide whether a machine "thinks": if a person conversing in writing cannot tell it apart from a human, the machine passes. More in the glossary → to tell Computers and Humans Apart". Modern versions, like reCAPTCHA, analyse the user’s behaviour to be less intrusive. The challenge for their designers is to stop bots without getting in the way of real people or harming accessibility.