Definitions of "ergonomics"
"Ergonomics" is a very silly word. The International Ergonomics Association says
Ergonomics (or human factors) is the scientific discipline concerned with the understanding of the interactions among humans and other elements of a system, and the profession that applies theoretical principles, data and methods to design in order to optimize human well being and overall system performance.
Practitioners of ergonomics, ergonomists, contribute to the planning, design and evaluation of tasks, jobs, products, organizations, environments and systems in order to make them compatible with the needs, abilities and limitations of people.
More concisely, they consider it the science (and the application of that science) that studies human systems to make them more productive and better for people. I suppose that's helpful for distinguishing it from "design" (whatever that is) but it's otherwise quite vague.
But I see ergonomics more as the acknowledgement that the scientific study of interactions between people and things can reveal much complexity.