CAPTCHA is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart". Websites often use CAPTCHA for anti-spam verification to make sure responses are being generated by an actual person and not a computer “bot”.
In practice, CAPTCHA consists of a simple test that the site can grade to assess if the respondent is a real person. These tests have to be difficult enough to stump a computer, but easy enough for most people to pass. This can sometimes present accessibility issues, because “most people” includes people with disabilities.
The most commonly used CAPTCHA present distorted text for the user to interpret. This can block people with visual disabilities because their screen readers can’t read it. (Remember that a computer not being able to discern the text is the whole point of a CAPTCHA).
Another alternative is audio CAPTCHA, to assure human verification is accessible to the blind and other visually impaired people. This then presents difficulties for a person with a hearing disability.
Our favorite type is logical CAPTCHA, providing logic-based questions instead of distorted images or audio to validate the user. For example simple math CAPTCHA can work as long as the numbers are not images, e.g. “2+2=?” Other options ask simple text-based logical question. Logical CAPTCHA is generally most accessible, allowing more people to receive all the information they need to be able to pass the test and use the website.
If you are using a system to develop your website, just search for “accessible captcha” and the name of your tool, and you should find some options. Also some web-development platforms have automatic ways to check for spam that occur in the background and don’t require any user interface.