My short answer is - I don't try to.
"What is, or are, ergonomics? What is, or are, Human Factors? If ergonomics and Human Factors are the same, then what is "ergonomics AND Human Factors?" These questions - and their answers - confuse people, and rightly so.
Human-Centred Design, on the other hand, enters people's vocabulary on one hearing. Generally, folk are pleased to hear it exists, and annoyed that it is not the norm in equal measure.
I practice communicating Human-Centred Design with the population at large by wearing the jacket in the picture. I forget about the writing on the back, so I am surprised when people in a queue ask me "What is Human-Centred Design?". I have got better at giving easily-understood answers. The guy in the chip shop was up for a long conversation on the merits of early Nokia phones (thank you Timo).
On my business card etc. I describe myself as a People-Systems Integrator, and this seems to be easily understood.
Ergonomics now tries to be a 'discipline' that does 'science' and a 'profession' that does 'practice', and the result is a mess. The explanatory logo at the International Ergonomics Association website has only one test label up front and high-contrast - Human Centered Design.
Most areas of work distinguish professional practice and underpinning science, e.g.
|Professional practice||Underpinning scientific discipline|
|Medicine||Medical research, immunology, physiology etc.|
|Software engineering||Computer science|
|1970's: Ergonomics||Ergonomics research, human sciences|
|2015 formal: Ergonomics||Ergonomics|
|2015 IRL: UX, HCD, IA, Ergonomics||Human sciences, social sciences, design thinking, Ergonomics|