What is meant by Captcha?

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