What is meant by Captcha?

In Archive by Fredy Ore

For a few months now I’ve been visiting a site called Word Spy, if you haven’t heard of it, it’s a great site by Paul McFedries which identifies recently coined words and phrases, old words used in new ways, and existing words that have enjoyed a recent renaissance.

I came across this word a little while ago and it was sitting in my Draft posts for a few weeks now among others.

Here it is:
(KAP.chuh) n. A computer-generated test that humans can pass but computer programs cannot.