The International Organization for Standardization (ISO) defines usability as the "extent to which a system, product, or service can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use" [1]. Basically, usability is a broad term that describes research on how effectively, efficiently, and satisfyingly a product or system can be used. Usability research can span from broad testing for functionality ("does it work as intended?") to the more focused universal usability research ("can users with specific needs use our product?").
While they may sound similar, "accessibility" and "usability" are not actually the same thing. For something to be accessible, it must meet bare minimum requirements (such as a ramp for a building). However, just because something meets the accessibility requirements, that doesn't mean it is also usable. Looking at the ramp example, often times buildings will technically be accessible because they have a ramp and a handicap door, but that ramp may be out of the way and hard to get to, difficult to find, or even too steep for most wheelchair users. "Universal design" is a term coined by the late Ron Mace and his colleagues at the Center for Universal Design at North Carolina State University. Universal design has 7 basic principles [2]:
Equitable use: the design is useful and marketable to people with diverse abilities
Flexibility in use: the design accommodates a wide range of individual preferences and abilities
Simple and intuitive use: use of the design is easy to understand, regardless of the user's experience, knowledge, language skills, or current concentration level
Perceptible information: the design communicates necessary information effectively to the user, regardless of ambient conditions or the user's sensory abilities
Tolerance for error: the design minimizes hazards and the adverse consequences of accidental or unintended actions
Low physical effort: the design can be used efficiently and comfortably and with a minimum of fatigue
Size and space for approach and use: appropriate size and space is provided for approach, reach, manipulation, and use regardless of the user's body size, posture, or mobility
While these principles were developed mostly for physical products, they can be applied to digital products and services as well. There are even guidelines out called the Web Content Accessibility Guidelines (WCAG) that help web developers understand how to make their websites more usable by disabled and impaired populations.
Usability researchers have a wide variety of tools at their disposal, depending on which research questions they want to answer. They can test how long a task takes to complete with the product (if the task can be completed at all), see how users may solve problems with the product, ask users questions about their opinions on the product via a questionnaire or interview, or even test the product in a real-life situation. One of the most common questionnaires used by usability researchers is the "Systems Usability Scale", commonly just called SUS. The SUS consists of 10 easy questions asking the user's opinions on various aspects of the product. Users rate each question on a scale from "Strongly Agree" to "Strongly Disagree", then the total score is calculated for the scale. Explanations and templates for the SUS are available here, try it out for yourself!
International Organization for Standardization. (2010). Ergonomics of human system interaction - Part 210: Human-centered design for interactive systems (ISO Standard No. 9241-201:2010, formerly 13407). https://www.iso.org/standard/52075.html
Connell, B.R., Jones, M., Mace, R., Mueller, J., Mullick, A., Ostroff, E., Sanford, J., Steinfeld, E., Story, M., & Vanderheiden, G. (April 1, 1997). The Principles of Universal Design. The Center for Universal Design, North Carolina State University. https://projects.ncsu.edu/ncsu/design/cud/about_ud/udprinciplestext.htm
World Wide Web Consortium (December 6, 2021). Web Content Accessibility Guidelines (WCAG) Overview. W3C Web Accessibility Initiative. https://www.w3.org/WAI/standards-guidelines/wcag/
U.S. General Services Administration (2021). System Usability Scale (SUS). Usability.Gov. https://www.usability.gov/how-to-and-tools/methods/system-usability-scale.html