For me...it's when you turn to someone else emotionally...to fulfill those emotional needs.
My fiance, at one point when things were really bad, did cheat emotionally. He met a girl who he talked to often, began telling her the things he used to tell me (things about his day, his work) hid their friendship from me (I didn't even know she existed) they were more than just friends, without every crossing a line physically.
He never fell in love with her, but did have a vested interest in her (that I don't even think he realized until we separated and he was able to see all that had happened) but people can and do fall in love with other people all the time, even if it hasn't reached a physical level...to me, loving someone else, is cheating. Confiding in someone else things you should only share with your significant other or used to share with them, Having a romantic, emotional bond with someone is cheating, to me anyways. And, honestly I think often times worse than physical cheating alone.
A one night stand has no strings attached, but if you're emotionally invested in someone, care for them, have feelings for them...that's a lot harder to walk away from.