Glossary
What is a CAPTCHA? Definition and how it works
Definition
A CAPTCHA is an automated test that a person can pass and a program is expected to fail. The name stands for Completely Automated Public Turing test to tell Computers and Humans Apart.
How it works
- The visitor completes a challenge in the browser, such as a puzzle, a checkbox or a game.
- The service returns a token to the page, and the page sends it along with the form.
- Your server sends the token to the CAPTCHA provider to check it before accepting the form.
Where it matters
- Signup, login, checkout and contact forms, where scripts submit at volume.
- Treat it as a cost control. Solving services and trained models can defeat many challenge types.
In CaptchaKit
CaptchaKit uses short arcade games as the challenge and a signed token that your server verifies with POST /api/verify.
Questions
What does CAPTCHA stand for?
Completely Automated Public Turing test to tell Computers and Humans Apart.
Who invented the term?
The term comes from a 2003 paper by von Ahn, Blum, Hopper and Langford, presented at EUROCRYPT 2003.
Is a CAPTCHA enough to stop bots?
No. Use it with a server-side token check and rate limits.
Source
- von Ahn, Blum, Hopper, Langford: CAPTCHA: Using Hard AI Problems for Security (checked 2026-10-18)
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