Accessibility is a general term used to describe the degree to which a system is usable by as many people as possible without modification. It is not to be confused with usability which is used to describe how easily a thing can be used by any type of user. One meaning of accessibility specifically focuses on people with disabilities and their use of assistive devices such as screen-reading web browsers or wheelchairs. Other meanings are discussed below.
Accessibility is strongly related to universal design. This is about making things accessible to all people (whether they have a disability or not). However, products marketed as having benefited from a Universal Design process are often actually the same devices customized specifically for use by people with disabilities.
In the project, accessibility covers not only the interface, but applies to concepts themselves. The project's view is that the Open Source paradign doesn't go far enough because the access to the knowledge contained within the source code is restricted to specialists who can understand the programming languages involved. Accessible knowledge means that the concepts can be rendered within the specific context of the recipient in question, reducing the need for specialists between the concepts and their use in the field.