Most Helpful Guy
Cheating is doing anything one person in the relationship considers to be a breach of your relationship. Mostly it's implied which can be dangerous and is why people should really communicate about what they feel is cheating.
Personally what I consider to be cheating really depends on the people involved and the situation. If she kissed someone that'd be cheating, but not if the person she's kissing is her mother or something. If she had sex with someone that'd be cheating, but not if there was a person pointing a gun to her head forcing her to.