I don't know if accessibility has ever been properly defined. I guess it is an impossible venture. For the most part, when we talk about accessibility, we tend to assume accessibility for people with disabilities. It is fair to an extent, because mostly these people suffer from the lack of accessibility. However, what accessibility really means, above usability, is that regardless of the circumstances, all people should be able to access something. A site, software, building, environment. But if we just want to talk about web technologies, in fact, if a visitor comes to your site who does not speak your language, it is an accessibility issue. Obviously, we cannot make web technology totally accessible to all. But within reason, we should aim to do so.