This introductory chapter provides a definition of magic. One of the most useful adjustments in the recent scholarship on magic has been the turn to considering magic as a dynamic social construct, instead of some particular reality. Magic is not a thing, but a way of talking. Thus, magic is a discourse pertaining to non-normative ritualized activity, in which the deviation from the norm is most often marked in terms of the perceived efficacy of the act, the familiarity of the performance within the cultural tradition, the ends for which the act is performed, or the social location of the performer. Such a discourse always has a history, since such a way of talking about things shifts over time as different people do the talking. When one speaks of “magic,” therefore, one should always explain: “magic for whom?” Any specific piece of evidence from the ancient Greco-Roman world provides an example of magic for that particular person, from one particular perspective. To speak of “magic in the ancient Greco-Roman world” is thus to refer to the whole range of things that various people in those cultures during those times could label as “magic.” The chapter then considers the act of drawing down the moon.