Is cheating while dating (as in having a boyfriend or girlfriend) known as adultery?
Having sex with someone you are not married to is called "fornication." See Notes on Sex for more information. Adultery is a type of fornication where one or both people are married, but not to each other.
Dating carries no formal obligations between the two people. It is only an indication that they might consider marrying the person they are dating, but have not yet made up their mind. Dating someone does not give you the right to have sex. Becoming engaged means you are intending to marry the other person, but even that does not give you the right to have sex. It is only when a couple enters into a covenant of marriage do they have the right to have sex. "Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge" (Hebrews 13:4).