: Web users may have disabilities which affect their ability to use web sites, including visual impairments, auditory impairments, motor disabilities, and cognitive disabilities. "Accessibility" is

Web users may have disabilities which affect their ability to use web sites, including visual impairments, auditory impairments, motor disabilities, and cognitive disabilities.

"Accessibility" is the practice of designing sites to accommodate such disabilities. For example, an accessible site always provides ALT text for images, so that the screen reading software used by blind or low-vision users can read aloud a sensible text-based alternative to the image.

In many cases, accessible design is mandated by law. For example, in the United States, institutions which receive federal funding are subject to the Section 508 standards, so-called after the amendment to the Rehabilitation Act which contains the standard.

There are also a number of technical standards related to web accessibility, notable the Web Content Accessibility Guidelines (WCAG, currently at version 2.0) and the Web Accessiblity Initiative's Accessible Rich Internet Applications standard (WAI-ARIA, currently in development).

This tag is appropriate for any question having to do with producing accessible sites, including questions:

about code
about the interaction between code and assistive software such as screen readers
about relevant technical standards
about best practices for conformance with applicable laws and standards

Specific legal questions are best referred to a lawyer, or to a disability support department (if you have access to one).

Links:

WCAG
WAI-ARIA
WebAIM, a good first stop for accessibility questions