Universal design, when used in conversation as a verb, is a process of designing something to be functional as possible for as many people as possible.
Universal design, when used as a noun, is an outcome of a design process, indicating that something is as functional as possible for as many people as possible.
Design that’s usable by all people:
The biggest challenge of designing for “all people” is making sure that it’s functional for people with any type of impairment of the body or the mind while ensuring that the design is useful for everyone.
Universal design is truly for everyone, despite widespread confusion about it only being for people affected by disability. Thing is, universal design wouldn’t be needed if disability wasn’t part of being human.
To the greatest extent possible:
Something that’s universally designed will work for as many people as possible, regardless of struggles with (1) upper body movement, strength, and/or sensation, (2) lower body movement strength, and/or sensation, (3) balance, (4) vision, (5) hearing, (6) cognition and memory, (7) activity tolerances, (8) speech and/or communication, (9) chemical sensitivities, (10) sensory tolerance, (11) needs for caregiver assistance, and (12) extremes in height and weight.
Without the need for adaptation or specialized design:
Adaptation is a process in which an individual has to change the way he or she typically interacts with something. Specialized design is for a specific demographic or need. Universal design is inclusive to any generation someone belongs to or a health condition that someone may have.
Note: yes, some people have significant functional needs that require specialized design. If universal design is the foundation for design, adding specialized features as-needed is much easier and more cost-effective than if a design is fully specialized.