It could be as simple as agreeing with one's partner to only eat unhealthy desserts in front of each other(*), and then sneaking a Frosted Pop Tart as a treat during an especially hard work day… Perhaps if it had been a quick breakfast purchase, it wouldn't even be "cheating" in context… But since it's a reward, presumably unrelated to actual caloric intake needs, it breaks the rule. And it's breaking the rule that's ultimately the bad thing, not which rule was broken.
To that end, I'd say that cheating is anything that steps outside previously-established boundaries, without prior re-negotiation of limits.
Where people tend to get into trouble, is that they don't treat such "small infractions" like the Pop Tarts as seriously and consistently as larger gaffes (kissing a coworker after a long post-work drinking session, say), and thus set up internal dissonance and misunderstandings about "the rules", and how bad it is to bend/break them.
Equally common, is just running a relationship based on mutual prior assumptions of what would be allowed, rather than creating the rules from scratch in a manner that creates an "optimal fit". In the startup world, the equivalent is trying to skimp on legal fees by stealing a competitor's TOS page from their web site, and find-and-replacing in a different company name. It might work for a while, and it's certainly a great strategy for a "doomed" effort, because it saves money/time/energy… But the more successful you are, the more a can of worms it becomes, and the harder it becomes to resolve before something truly bad happens.
In the relationship coaching I do, and the singles I see, both of the above trouble spots represent the bulk of frustration with a struggling relationship, even when no one is "breaking the rules". Ideally, rather than relying on their own preconceived notions, or even the definitions of others like me, each active member of a relationship should create, communicate, and collaborate on their own definition of and boundaries around cheating, and then stick with that.
If that sounds like too much work for something that should "just be understood, based on traditional mores", then you may be wise to also adopt another common tradition: turning a blind eye to cheating for the sake of the relationship.
Now does all that work I mentioned really still sound so bad? 😉
(* For those in relationships looking to lose a few pounds, this works pretty well if you can stick to it.)