At the point where anything is happening that your partner would disapprove of, or that you wouldn't want THEM doing with someone else. Interestingly, those boundaries can vary from couple to couple.
Before saying what cheating is in my opinion, I must address that this issue is one determined by each couple differently. Everyone has their own idea of what cheating is to them. In one of my past relationships cheating was different than it is in my current relationship.
The point at which it becomes cheating is when your intention or the other persons intention is no longer just friendly. If you are just talking, but you are talking about cheating or talking about doing things to each other that would be considered cheating, then its cheating. Usually if you feel like you are cheating you are. Its about your intention. People will say, "hey what about fantasizing?" That is thinking about having sex with someone else, but it is an internal thought that is not being acted on. If you are role playing fantasies out with a friend, even if there is no touching its cheating.
Another aspect of cheating is acting in such a way as to hid your actions/activities from your SO. If you think you need to lie to your SO about what you are doing with that other person, you are probably cheating.
In one relationship I had, it was okay to have full intercourse with other people. Even okay to have other girl/boy friends. However, the SO had to know the person and approve of the relationship. All parties knew the truth of the situation and were comfortable with it.
In the current relationship I am in, if he went over to some girls house and didn't tell me about it and then I found out about it later I would be convinced he was cheating on me, even if he wasn't. The fact that he felt the need to hid his plans to hang out with said girl, tells me a lot about his intentions. If he told me he was going over to a certain girls house, prior to going I would not worry about it at all. I find that when someone lies to hide something from me, they can try and defend their actions til they are blue in the face. It wont matter to me, because they already lied to me. If they will lie to me about one thing, how can I trust what they say? I can't.
So, if you have a friend that you are hanging out with and it truly is just friends and you are not doing anything you feel the need to hide. Just make sure you are honest about going to visit the person and make sure your SO knows they are welcome to join you if they would like. If your SO catches you in a lie that can be more harmful to your relationship than any other thing.
The open relationship I was in, my SO wanted to date someone he worked with, we talked about it and I was not okay with it. He said that he was just going to hang out with her as friends and I was fine with that. Then I started catching him in lie after lie. Our divorce was finalized in May. I forgive him for almost everything, but I can never forgive him for lying to me. We were married for 8 years.
when there is a hidden desire behind it.
Hmm.... body contact.kissing... and things you normally would do only with your SO.