Most Helpful Guy
For something to be 'cheating', it has to include a degree of dishonesty. Holding hands with a guy may be cheating in a lot of cases, but if your boyfriend knew you held your best guy friend's hand sometimes, and was ok with it, it wouldn't be cheating. It's not about the act, it is about the intent, and the dishonesty/secrecy around it.
If you are doing something (not you specifically, you in a general sense) that you think would upset your partner if he found out, it is mostly likely cheating. For some people, that could be texting a guy that you met at a party, for others that wouldn't be an issue. It could be talking to guys on facebook, it may not be. It is context sensitive. It has to be taken in the context of the relationship it is referring to. What boundaries were set for that relationship. What expectations were set. And did the person doing the cheating know that they were going outside of those expectations.
So, I suppose it could be summed up as 'knowingly acting in a way that violates the established boundaries of the two partners in a relationship'.
Most Helpful Girl
An emotional attachment that goes beyond a friendship. If the connection you have with another goes past what you would talk about with a friend and you aren't telling you SO those same things, then you are cheating on them. You have choose to take part of your self and give it to another and keep it from you SO. I also consider any kind of lying.