CAPTCHA definition

Etymology

Coined by a team of researchers at Carnegie Mellon University in 2000 as a loose
of "Completely Automated Public Turing test to tell Computers and Humans Apart".

Noun

A distorted image of letters and numbers used to prevent automated use of websites.

I find CAPTCHAs very difficult to read.
Anything that a human must do to prove that they are not a computer.

Usage notes

Although occasionally spelt with lowercase letters (captcha), or with an initial uppercase letter (Captcha), most authors remain aware of its origin as an acronym and use uppercase.