Cheating can be described as anything from sharing a kiss to engaging in a form of sexual activity to sharing an emotional connection with a person outside of your relationship. No matter how you define the term, its definition essentially means the same across the board: sharing your mind, body or spirit with a person other than your spouse or romantic partner in such a way that the activity could be perceived as a threat to your relationship and must be kept hidden in order to be maintained. Popular television host, Dr. Phil, put it best when he said, "Cheating is engaging in any activity that you would not otherwise engage in if your spouse were present."
eHow Article: Facts on Cheating