You are right in that it varies from person to person. Someone having sex via webcam or "sexting" (not ever meeting) the other person is considered cheating to a lot of people. Others also believe there is a such thing as "emotional cheating" (chat room affairs). Naturally having sex with other would not be considered cheating if one is an "open relationship" or a couple consider themselves to be "swingers". The mistake a lot of people make is assuming everyone has the same definition of "cheating" or "betrayal". It's always good to talk about this subject early on before one becomes to emotionally invested. The sooner you know whether or not you are on the "same page" the better!
In my opinion "cheating" always (involves lying directly or by omission) in connection with something you know your mate would not approve of.
This could range from flirting, using sexual innuendo, secret meetings, exchanging love notes all the way up to having sex. I also agree with Dr. Phil's answer to the question.
"If you are saying or doing something with someone that you would NOT do if your mate were sitting right next to you then you are probably cheating."