The reason the definition gets murky is because people try to define it with external observations when it's largely an internal event. Emotional cheating, at its core, is just fostering romantic feelings for someone other than your romantic partner. Simple. You can't read your partner's mind, though, so you rely on things like how secretive they're being, how much time they spend with someone else, what they talk about, flirtation, etc. But just because you don't see the proof of it--just like even if you don't see your partner fucking someone else--it doesn't mean it's not happening. And just because the person cheated on has to rely on flawed measures to figure out what's happening doesn't make their feelings on it any less valid.

Of course, any definition can be blown out of the water by people's own personal boundaries for their own relationships. Not everyone considers sexual or romantic relationships with other people cheating, but I think we're both on the same page that we're talking in general.

Also, disclaimer, I obviously got some feelin's about this, but I'm not trying to be argumentative just for the sake of it. It's just something I had to think long and hard about, so I hope I'm not being rude.