What do you consider "cheating"?

just wondering how far it takes someone to go for you to think that they cheated. details and explanations pls... thanks =) any PLEASE SOME1 answer this question already! http://answers.yahoo.com/question/index;_ylt=AqJdk63K06MWMwaNV2pBmLzsy6IX;_ylv=3?qid=20090320203817AA8cKCQ
Update: i do mean relationship cheating
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