What is "Cheating?"

During another one of our tipsy-making girls' nights, conversation turned to "so what constitutes cheating exactly?" Thankfully, we didn't talk about this for long, as I had pressing cat barf stories that I knew my friends needed to know about.

Is a kiss cheating? Is thinking about kissing someone cheating? How much thinking, exactly? What about the work buddy of the opposite sex that you hang out with, after hours, a few times a week? Whether you're sweet on him or not, you aren't home with your bethrothed are you? But is that cheating, or is that just mean? Obviously, you're a cheat if you have sex with someone outside of your relationship. Phone sex, online sex, thinking about asking about phone sex with someone else.... cheating or not? It isn't black and white, is it?

So I ask you: where is your line? At what point does "bad judgement" become "cheating" in your mind (or heart)?