No, you cannot cheat on someone if you are not dating them. You cannot cheat on anyone. If you previously dated someone and have since ended the relationship, it must be resolved before you go on to someone else. Perhaps they still harbor romantic sentiments for you. Or perhaps they did not feel like they could be completely honest with you about their feelings. If you are both single and want to date then yes, you can date each other without either of you being committed to the other. However, if you aren't looking to get serious then there is no need to label yourself as "just friends". Keep in mind that this person will not be expected to do any of the following if you are only friends: pay for dates, meet your needs by giving you emotional support, or fill the role of a boyfriend/girlfriend. The only people who can tell if you are cheating on someone are those people. Otherwise, you're free to love whoever you want with no consequences other than the risk of getting caught.