new1...
having been in a situation close to this the best advice i can give is...
Stop asking for a definition of "cheating" here. You are going to get 1,000 answers on basically 2 (maybe 3) sides.
We can't and shouldn't validate any emotions for your or excuses for her. If that is how you feel then that is a valid emotion to you. Just because myself, or Oz, or Joe Blow may not agree with it won't change your feeling on it. And just because we may agree with her won't change your hurt or anger over it.
If it is a relationship worth saving then what you need to do is get her into counseling with you to work out the issues on it. Such as:
"why did she feel the need to do that?"
"Was it sexual based or emotionally based?" (yes, there is a difference)
"how can we prevent this trap in the future?"
"how can we rebuild the trust?"
Use a natural party, like a counselor, to help determine why what happened did happen and how to move forward either together or separate from there. If you decide to try perhaps a good book for each of you to read is Do I Have to Give Up Me to Be Loved by You? by Jordan and Margaret Paul
It doesn't matter what we think... trust me, I could give you "instances" on both sides, for and against. God knows I have played that game enough time. Bottom line though, it is your life and you have to determine your next course of action.
Don't mean to sound harsh but asking here won't help you figure out the next step or move forward.
Hugs! Good Luck... I know it is rough.