"Web acessibility means that people with disabilities can use the web. More specifically, web accessibility means that people with disabilities can perceive, understand, navigate, and interact with the web, and that they can contribute to the web. Web accessibility also benefits others, including older people with changing abilities due to aging.
"Web accessibility encompasses all disabilities that affect access to the web, including visual, auditory, physical, speech, cognitive, and neurological disabilities."
W3C definition of web accessibility