Accessibility
What is accessibility?
Digital «accessibility» relates to designing content (on the web or otherwise) so that it can be used by everyone, including those with (variable) limitations of the following capabilities:
Visual: blindness, low vision and color blindness.
Motor: reduced ability to use a mouse, slow response time, limited fine motor control.
Hearing: profound deafness and hearing problems.
Cognitive: learning difficulty, easy distraction, inability to focus on large amounts of information or data.
Accessibility is a facet of usability. Unless you deliberately alienate users with such limitations, you cannot claim your user experience as usable until it is accessible.
Source: A is for Accessibility: 12 tips for designing an inclusive user experience
Other references
Accessibility Interaction Design Foundation