Cheating??

In a relationship what really is "cheating"? In my last relationship my ex told me that she kissed her ex boyfriend when me and her were still dating. She said it was just to find out if she still had feelings for him, I thought of this as cheating. I know it was just a small gesture but I feel like any breech of loyalty or slip of trust is cheating, even if it's as small as one kiss. What do you believe is really "cheating"?