See also #usable.
There is a W3C standard on this topic
Capability of a product or system to be usable by people with the widest range of characteristics and capabilities to achieve a specified goal in a specified context of use.
Note: The range of capabilities includes disabilities associated with age.
Derived from ISO-25010-2011
Accessibility is the design of products, devices, services, vehicles, or environments so as to be usable by people with disabilities.
Accessibility is the concept of whether a product or service can be used by everyone—however they encounter it.