CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." It is a security measure used on websites to determine whether a user is a human or a bot. Typically, it presents challenges like identifying distorted text, selecting images, or solving simple puzzles that are easy for humans but difficult for automated programs. The primary purpose of CAPTCHA is to prevent automated software from abusing online services, such as creating fake accounts or submitting spam. By ensuring that a real person is interacting with the site, CAPTCHA helps maintain the integrity and security of online platforms.