CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart): a challenge-response mechanism used to distinguish human users from bots.
Common Types:
- Text-based – distorted characters a user must type.
- Image-based – select images matching a category (traffic lights, crosswalks).
- reCAPTCHA v2 – checkbox (“I’m not a robot”) backed by behavioural analysis.
- reCAPTCHA v3 / invisible CAPTCHA – no user interaction; scores traffic based on behaviour, runs silently in the background.
- Audio CAPTCHA – spoken challenge for accessibility.