Definition:

Accessibility is one of the major tenets of design, and covers almost every design practice. Accessibility is not just whether or not someone with a disability can use a design effectively. It also covers the ability of a design to have mutual benefits for diverse groups of users. Can both a new user and an advanced user both accomplish their own specific goals?

Interaction Design:

In interaction design accessibility focuses on two very different types of design problems — human needs and technical limitations. It is a well known principle of accessibility to account for color-blindness, users with poor eyesight and other physical disabilities. Technical limitations due to aging browsers, lack of Javascript and slow internet connections are starting to become just as important within web application accessibility. Flickr's sign up button has a few issues with color blindness — doesn't the button on the right look disabled?

User Experience Design:

In user experience design, accessibility also contains a human element and a technical element. The Rackspace Cloud's current signup process requires a phone-based fraud check. What if our new user is completely legitimate but can't speak any of the languages that Rackspace representatives do? Can a 6'8" man realistically fit into his new Porche? The accessibility of a Porche's cockpit is a technical limitation that was designed to balance mechanical systems, aerodynamics, and the experience of driving the car. At some point, the there is a limit for each of the variables, and a decision was made to limit the physical size to increase the overall experience.