From Wikipedia article on accessibility:
Accessible course materials are able to be accessed by the widest possible range of users—including people with special needs or disabilities—or can be accessed through the use of assistive technology.
From Michigan State University
Is there a legal requirement that University Websites be accessible?Yes. Title II of the Americans with Disabilities Act ("Title II") and Section 504 of the Rehabilitation Act of 1973 ("Section 504") require that public universities provide qualified individuals with disabilities equal access to their programs, services, and activities unless doing so would fundamentally alter the nature of their programs, services, or activities or would impose an undue burden. As part of that obligation, public universities are required to provide equally effective communication to persons with disabilities, regardless of whether the university generally communicates through print media, audio media, or computerized media such as the Internet. Universities that use the Internet for communication regarding their programs, goods, and services must make that information accessible.
Below is a list of resources to help you create course content that's accessible to as many people as possible.
www.webaim.org: information, training, resources, guidelines and standards for Web accessibility
A one-page Cheat Sheet PDF on creating accessible content from National Center on Disability and Access to Education (NCDAE)