CAPTCHA

A CAPTCHA (/ˈkæp.tʃə/ KAP-chə) is a type of challenge–response test used in computing to determine whether the user is human in order to deter bot attacks and spam.[1] The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford.[2] It is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart."[3] A historically common type of CAPTCHA (displayed as Version 1.0[clarification needed]) was first invented in 1997 by two groups working in parallel. This form of CAPTCHA requires entering a sequence of letters or numbers in a distorted image. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, CAPTCHAs are sometimes described as reverse Turing tests. https://en.wikipedia.org/wiki/CAPTCHA

Greg Kennedy: If you ever need a #captcha on your website I can't stress enough how effective it is to just use "answer this easy question" type validation on signup. I ran a wiki for a bit and within a week or two got slammed by bots that could solve captchas and get through email validation just fine. As soon as I added a simple "what's the name of (thing)?" box, where a hint to the answer could even be found on the same page!, I never had another fake signup. I'll probably never run a website with user signups again without this feature from the start. It really was that effective.