In an arbitrary sense of the word.
When is cheating not "arbitrary?" Different people define their relationships differently.
Is it only cheating when it is straight-out heterosexual intercourse? Some join swingers clubs, are they "cheating" on their spouse if they sleep with another member of the club, if their spouse is being mounted by some other stranger at the same time? Is that "cheating," exactly?
Cheating is only what is defined by the participants in a relationship as "cheating." Unfortunately, sometimes different people in a relationship have a different idea of what exactly constitutes cheating, and they don't always sit down and discuss everything, which can lead to problems down the road. They just assume the other person has the same idea they do...