Ergonomics is a scientific study of the interaction between people and a non-natural environment.
Applied ergonomics is the use of the knowledge of ergonomics when designing or adapting utensils, information systems and working environment.
The word Ergonomics is derived from Ergon (work) and Nomoi (nature laws) or Nomos (Laws).
The American term "Human Factors" is somewhat clearer.
Of course, usability has been taken into account for a long time when creating utensils, but not in a scientific way.
In 1857 the Polish researcher B. W. Jastrzebowski laid the foundation for what is now Ergonomics, but it took until 1949 before it became a real scientific discipline.
Ergonomic is again derived from ergonomics, which generally means taking into account "the ergonomics of something".
An "Ergonomic advice agency" sounds nice, but is linguistic nonsense.
That means: products with a design based on ergonomics.
Unfortunately it is often used to sell some strangely designed mice and keyboards for a lot of money.
Ergonomics or common sense
In many cases there is little distinction, but ergonomics does add something important, the knowledge gathered by scientific research.