How do you define infidelity (cheating)?
Having an affair?
Hooking up after a drunk night?
Paid sex with an escort/prostitute
Happy ending massage?
French kissing someone?
Staying active on a dating site?
Flirting?
Sexting?
If it's something you do with someone else other than your partner that you need to keep secret because of fear of him or her finding out AND getting hurt/heartbroken, then it's cheating.