Cheating is robbing your partner of an experience or item that is desireable and doing it in a manner that seeks to conceal the happening. It could be sneaking off to eat a hamburger or having sex and lying about it to your partner. Obviously the degree of damage to the relationship is going to be determined by the degree of broken trust.
In an open relationship trust is more easily broken, though completely unnecessary. It is absolutely possible for me to cheat on my husband even though we have a completely open relationship as part of his cuckolding fantasy is hearing about and seeing the aftermath of the experience! Now it wouldn't be an "ULTIMATE" betrayal because he has a bit of a betrayal fetish mixed into his sexual makeup but it would be damaging none-the-less. The only thing I could do to ruin our relationship beyond repair is to leave him....abandonment is NOT a kink for him, though oddly enough it is a bit of a kink for my life partner. It would be a massive betrayal of my life partner if I were to lie or sneak around as he is a very open and honest person. He wants me to be happy and loves celebrating with me whatever happens...should anything happen. I am such a disappointment to my cuck! I just don't have casual sex often enough for him. Sometimes he almost feel like this is a form of cheating but since he doesn't think it's entirely rational to feel 'cheated' by a wife who simply places more value on sex than he does he contents himself with the occassional grumble.

That is a great question JR for many people cheating is a component of the cuckolding experience...indeed it was the word for cheating in most countries for the longest time.
The word refers to having a child and claiming it to be the spouses offspring...like a cuckoo laying an egg in another bird's nest but then so is the Biblical definition of adultery. The act of having a son and claiming the man you are married too is the father when he is not and without his permission is the definition of adultery...and also cuckoldry.

Historically an older man would marry a young woman and with or without his permission she would have numerous sexual affairs earning him the title of "Cuckold". If he was sporting he might wear antlers of deer or moose to proclaim how "horny" he was....if he was not then she could suffer horrible consequences! This was such a large "joke" in times past that am man merely had to walk into a room with antlers on his head to start the knowing chuckles or leers. The obvious ridicule of the pagan practice of dressing in deer antlers to commune with Herne (and other antlered Gods) aside, this was a pretty common joke during the middle ages.

In our modern times I think it really comes down to the couple and their lines in the sand, so to speak. Cockolding is a term meant to be synonymous with cheating but like any word can have several definitions as time progresses. I guess if you like the term and are comfortable with the practice then cuckold might be a sexy way of describing a man who is sexually monogamous with a woman who is not....AND could possibly be used to describe ANYONE who is sexually monogamous with a non-monogamous partner. Take away the negative connotations and you simply have a label.