Pardon me for necroing an older thread, but I think this is a very interesting discussion.
I don't think there is so much a difference in the definitions of the various words mentioned here so much as understanding what those mean in terms of actions.
"Cheating" is the obvious one that springs to my mind, but "love" and "commitment" are two others.
The concept of commitment to most monogamous people that I have spoken to implies committing to one person completely. To a poly person it means something different.
When the poly community itself can't agree on common definitions and applications of language, then we can't really expect that to be done when communicating with non-poly people. The more I have read about the discussions pro and con coming up with some sort of common definitions for terms, the more it feels like The Tower of Babel (and one or two who fancy themselves as Nimrods).
If there are no ambitions to get so-called mainstream society to become more accepting of some sort of poly lifestyle, then I think we can carry on as we are - in a state of flux and chaos. But most emerging sub-cultures have in the past come up with a coherent language that they use to communicate within the sub-culture and then to the wider world. I would postulate that this has pretty much been a pre-requisite for emergence and stability.
We are a long way from that, I fear, and not getting much closer.
"Listen, or your tongue will make you deaf." - Native American Proverb