http://en.wikipedia.org/wiki/Cheating
Cheating constitutes doing anything, whether verbal or physical, that one would not do in front of their significant other. Such examples would include: expressing attraction to another person, talking, electronic communications, texting, data, kissing, making out, and sexual contact.[citation needed]
Many people consider cheating to be any violation of the mutually agreed-upon rules or boundaries of a relationship, which may or may not include sexual monogamy. For example, in some polygamous relationships, the concepts of commitment and fidelity do not necessarily hinge on complete sexual or emotional monogamy. Whether polygamous or monogamous, the boundaries to which people agree vary widely, and sometimes these boundaries evolve within each relationship
Most of this has already been said here, and I agree with it. From the same page, the more general definition of cheating is:
breaking of rules to gain advantage in a competitive situation. The rules infringed may be explicit, or they may be from an unwritten code of conduct based on morality, ethics or custom, making the identification of cheating a subjective process.
I guess, for me, cheating in a relationship is when one partner does something behind the scenes that breaks their fidelity rules. If the do the same thing in view of their partner, it's not cheating, it's simply infidelity of one sort or another. Of course the suspect might claim they didn't understand what the rules meant in terms of their specific behaviour, and unless it can be shown that they have actively tried to hide what they were doing from, it could be impossible to know whether it was an innocent mistake or not. And in a new relationship, it might take a parter some time to get used to the significance of their new role in day-to-day terms.....they might declare undying love for you but not realise what that means, especially if they tend to love in a narcissistic way and have been partnerless for a long time, or partnered but with looser mutual expectations, which is what a lot of younger couples seem to opt for these days. They might simply not know themselves well enough to be immediately capable of keeping to the agreed contract, they might not be well-versed in their understanding of emotional infidelity and danger signs, they might misjudge their sexual vulnerability to others, be surprised when it goes further than they'd expected, and be afraid to 'fess up about their mistakes. That's rather less culpable than deliberate cheating, but I guess it's a form of cheating nonetheless.
I also believe that cheating can be done entirely in the mind - crushes on other people while in a supposedly monogamous relationship, thinking of others while having sex with a partner, indulging in amorous or sexual thoughts about somebody one found attractive, rather than nipping the idea in the bud, directing one's thoughts into more innocent avenues, and taking on board the idea that the mind may be wandering because of problems in the relationship, and redoubling one's efforts to sort that out.