You make it sound like there is only one definition that everyone naturally becomes aware of.
Looking to the dictionary tells me that cheating involves being "sexually unfaithful", but even that is still open to interpretation. I know some people who believe it only takes a "thought" to constitute cheating, while others require some physical act, but even then it's fuzzy exactly where the line is drawn. It's especially more difficult to pinpoint the exact moment of cheating amongst members of the same sex, as in our society it's possible for total strangers to become aware of each other's bodies in ways that would bring suspicion were they opposite sexes (e.g. open showers, locker rooms, etc.).
If a thought consists of cheating, that's some crazy relationship.
It's okay to look at other people and admire their attractiveness and/or beauty. I mean, we do it with celebrities all the time. The problem comes when people start purposely flirting or getting all touchy with others.
Otherwise, looking at other people and playfully flirting every now and then, it's completely fine, IMO. Your significant other is a person, not a slave. To forbid him or her of finding other people attractive is just crazy.
What you want is for him or her to be faithful to you (both romantically and sexually), if you're monogamous, that is. But you can't limit one's imagination.