What is Cheating?

In another thread, someone asked questions about what should be considered "cheating" and I thought this deserved a separate discussion.

When does flirting become cheating?

Is it cheating to "sext"?
Is it cheating to pretend to be single or hide your relationship?
Is it cheating to send someone an explicit photo of yourself?
Is it cheating to masturbate using porn?
Is it cheating to go to a strip club or sex show?
Is it cheating to have an online dating profile?
Is it cheating to add an obvious admirer on Facebook?