Besides an actual affair here are a few things that could be considered cheating.

A lapdance - this depends on where you are in the relationship. Are you having trust issues? If your partner spends a lot at the strip club or if things have happened during those lapdances that could be upsetting. It's NOT cheating if you know about it or are there.

Contacting exes - If this is done secretly and for some purpose other than to catch up it gets a little iffy.

Flirting online - This is a big red flag because it involves planning and preparation, which spells trouble!

Flirting offline - Everyone does this from time to time with the office girl, waitress and the cute checkout girl.

Emotional affairs - This is by far the worst form of cheating! In surveys women were more likely to forgive a physical affair over an emotional affair.

Sexting - Why would you need to text someone these things if you didn't have a motive or if they involved things like, "I can't stop thinking about you."

I think all these things depends on where your relationship is and if these things are grey areas of cheating for you.