Prosperity vs Poverty

I just watched a short film that got me to thinking about this: In a relationship what defines cheating at its core? I imagine this probably varies by person, but I think, at its very core, cheating is anytime you give elsewhere what you should be giving to the relationship. (I know taking the logic to it's end, this could become a huge mind fuck, so don't do that.)

I'd be interested to hear a different thought. No judgement, just inquiry. (I'm supposed to make you know, this has nothing to do with any relationship I'm in or know of.LOL!)