CAPTCHA (stands for "Completely Automated Public Turing test to tell Computers and Humans Apart") is a sequence of alphanumeric characters in twisted, weird, or melty fonts that are hard to read and that you must type into a little box when you are ordering something on-line. I've seen them for years, but didn't know what they were called until someone told me a few months ago on this very forum. And on that day I also learned that they were invented by an autistic mathematician, Alan Turing. Gotta love that irony.
They are sometimes hard to read, but I can usually decipher them.