Accessibility focuses on how a disabled person accesses or benefits from a site, system or application. By making your website accessible, you are ensuring that all of your potential users, including people with disabilities, have a decent user experience and are able to easily access your information.
-https://www.usability.gov/what-and-why/accessibility.html
Universal Design is the process of creating products that are accessible to people with a wide range of abilities, disabilities, and other characteristics. Universally designed products accommodate individual preferences and abilities; communicate necessary information effectively (regardless of ambient conditions or the user's sensory abilities); and can be approached, reached, manipulated, and used regardless of the individual's body size, posture, or mobility.
-https://www.washington.edu/doit/what-universal-design-0