Universal Design is the design of environments, artifacts, and information in such a way that they are accessible to all. Critical to the practice of universal design is the recognition that designing for accessibility is not a niche addition for a small minority, but simply a facet of good design. Designing to include the broadest array of people almost almost improves design even for those who may not currently identify as a person with disabilities or adaptive needs.
From the Centre for Excellence in Universal Design:
Universal Design is the design and composition of an environment so that it can be accessed, understood and used to the greatest extent possible by all people regardless of their age, size, ability or disability. An environment (or any building, product, or service in that environment) should be designed to meet the needs of all people who wish to use it. This is not a special requirement, for the benefit of only a minority of the population. It is a fundamental condition of good design. If an environment is accessible, usable, convenient and a pleasure to use, everyone benefits.
A powerful example of universal design is the curb cut Curb cuts, the small ramps or "cuts" in the sidewalk at almost every modern street corner, are generally taken for granted today. Before the rise of the disability rights movement in the 1960s, curbs on corners were generally raised about six inches off the street, making every block a barrier to those using wheelchairs. Between 1970-1974, largely in response to advocacy by the Independent Living Movement, Berkeley created the first accessible wheelchair route in the United States, and by 1990 the passing of the Americans with Disabilities Act mandated curb cuts on all street corners.
Curb cuts are not only useful for those with wheelchairs. They make significantly easier passage by those with strollers, wheeled luggage, shopping carts, and other equipment. Similarly, altering environments or artifacts for easier use by groups with specific adaptive needs tends to make them more usable by all, and often to open up new and better affordances.