Accessibility is a general term used to describe the degree to which a product, device, service, or environment is available to as many people as possible. Accessibility can be viewed as the "ability to access" and possible benefit of some system or entity. Accessibility is often used to focus on people with disabilities or special needs and their right of access to entities, often through use of assistive technology.
Questions having the tag accessibility regards rules, guidelines, design and implementation of accessibility in the context of User Experience.
Reference: Wikipedia - Accessibility