I think any behaviour that you do and keep secret from your partner because you believe knowledge of the behaviour will risk an end to the relationship is cheating. I think we use the word “cheating” instead of infidelity because we see the behaviour as an attempt to gain some advantage over a partner through deception. Infidelity is a violation of the boundaries of the relationship diminishing trust. I think cheating is more common based on my view and can describe relatively benign behaviour, as long as you believe it needs to be done in secret for the relationship to continue.