The function of a CAPTCHA in online security is to verify that a user is human and not an automated bot. It presents challenges such as distorted text, image recognition, or basic math problems that are difficult for machines to solve but relatively easy for humans. By implementing CAPTCHAs, websites can prevent spam, unauthorized access, and brute-force attacks that exploit automated scripts. This enhances the overall security of online services, ensuring that only legitimate users can access certain features, submit forms, or perform transactions. CAPTCHAs play a critical role in maintaining the integrity of online platforms.