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?
Anything that you wouldn't tell your partner. To me, successful relationships are built on honesty and respect - it is up to you and your partner to define infidelity between the two of you.