I'll add to Sheri's post by saying that the romantic interaction can be emotional, not just physical. In addition, the contact can take the form of a lap dance or contact that leads to ejaculation - like the d.c. madam prostitutes that provided a sexual fantasy visually so the men could get themselves off.
Cheating, to me, is any sort of romantic or sexual interaction with someone who's not your SO, whether it's in person, online, by phone or whatever.
Sheri