Accessibility is a general term used to describe the degree to which a product (e.g., device, service, environment) is accessible by as many people as possible.
Accessibility can be viewed as the "ability to access" the functionality, and possible benefit, of some system or entity.
Accessibility is often used to focus on people with disabilities and their right of access to entities, often through use of assistive technology.
Several definitions of accessibility refer directly to access-based individual rights laws and regulations. Products or services designed to meet these regulations are often termed Easy Access or Accessible.
Accessibility is not to be confused with usability which is used to describe the extent to which a product (e.g., device, service, environment) can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.
Accessibility is strongly related to universal design when the approach involves "direct access." This is about making things accessible to all people (whether they have a disability or not). -wikipedia-