Infidelity is defined very broadly as anything that breeches intimate relationship contract. So that, if you and your spouse agree to be sexually monogamous and to be secretive about your sex life and then you go on and tell your attractive male co-worker all about how your husband ejaculates too fast, that would be considered infidelity. By sharing intimate information with outsiders, you not only breech the contract and violate the trust, you betray the person who trusts that you won't do that.
You also invite these strangers into your relationship. So you have to be very careful about what kind of confidences you share with outsiders. There has been a definition of this as windows and walls. You have to put up the walls that safeguard the intimacy of your relationship and decide where you want to put windows and what kind of people you're going to open these windows to. Therefore, infidelity has a very broad definition and is not limited to sexual contact, but includes sharing of any intimate information with those who are outsiders to your relationship.
By the same token, if you and your partner discuss your relationship and decide that you want a more open relationship that would include things like talking to people on the internet, maybe sexting, maybe going into chat rooms, then by you doing it you wouldn't be violating your relationship contract and it would not be considered cheating or infidelity.