giving your heart, body, mind and soul to someone, other than your committed partner..
If you have to ask you went to far already.
Not following the rules, the rules you agreed to follow. so whether you're cheating on a test, speeding as you drive, cheating on a lover, they are all the same thing. your not following the rules as you agreed to do initially.
Well if your into the biblical thinking , To even look at another with lust in your heart is considered a sin (Cheating)
I suppose the factors lay in honesty and gaidens in the parties, one person is doing wrong, the other is doing wrong as well, there is no right in this one story called "cheating". Someone needs to confess and enact integrity in relationship commitments...or there is nothing but a facade in both faces.
Insecurity or comittment issues.
Looking through the Scrabble bag for the Q and the U