Web accessibility refers to the action of making website usable by disabled people (notably visual impairment). Web accessibility can be obtained in part by way of semantic HTML coding.
In a broader sense, web accessibility can also designate Internet technologies accessibility to the majority and notably to economically underprivileged global populations. As part of this, web accessibility is an economic and social problem.
In those two domains of web accessibility, W3C standards are instrumental.
Web accessibility can also designate the capacity of a webpage and its contents to be accessed by search engines spiders. See on this topic: spider firendly.
The W3C Web Accessibility Initiative: