"...the science of work: of the people who do it and the ways it is done; the tools and equipment they use, the places they work in, and the psychosocial aspects of the working situation." Pheasant, 1996
That is just one of many definitions of ergonomics, or 'human factors' as it is also known. But it is not just about people at work; ergonomics applies scientific information about people to the design of objects, systems and environments which are used by them. Ergonomics comes into everything which involves human use and ideally should always be considered during design and even when buying and / or selecting items for use in or within the workplace or by the public.
Ergonomics is not just about the science, it is also very much about the application, ensuring that problems are looked at from various perspectives in a systematic manner.
Usability is an essential aspect of ergonomic design - it refers not only to the ease with which a user can use or interact with a system or object, but also to the principles and methods used to design and assess an object's 'user-friendliness'. This might be for a specific group of people, or it might be for everybody (as far as possible). Of course, usability also incorporates 'accessibility' so that capabilities and limitations are taken into account as a normal part of the design process.
The Ergonomics Society
Ergonomics 4 Schools
International Ergonomics Association