It stands for "completely automated public Turing test to tell computers and humans apart"; no, the acronym doesn't really fit.

它是“completely automated public Turing test to tell computers and humans apart”的首字母，意为“区分人与计算机的全自动公众图灵测试”；当然，首字母缩写并不是完全对应的。

youdao

Captcha stands for Completely Automated Public Turing test to tell Computers and Humans Apart, and was first developed at Carnegie Mellon university in 2000.

BBC: Ticketmaster dumps 'hated' Captcha verification system