If you had registered for a new email account in the last couple of years or signed up for a service online,
you might have encountered a form where you had to accurately reproduce the content embedded in a
distorted image onto a field before submission. The purpose of this step is to prevent bots from taking part in online polls, registering for free email accounts etc.

Though I have interacted with this process for years, I stumbled on the definition only recently. CAPTCHA is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart". More on this term here.