In some forms of BDSM the role of master means one who dictates to and controls another person, who is known as a submissive or slave. A master is a dominant, but the term implies a more pervasive relationship than "dominant" does. A master is usually a master to a specific other person or persons. A master/slave relationship suggests one that goes beyond sexual scenes, and affects the day-to-day life of the people involved.
However, different people have different conceptions of the role, and may use it in different ways. There is no single accepted and definitive list of traits that a "master" must possess, nor is there clear agreement on what distinguishes a "master" from a "dominant".
The term "Master" may be used as a term of address, either alone ("I wish to speak with you, Master") or prefixed to a name ("Master Michael").
In non-BDSM contexts, a master may be a person who has achieved a high level of skill is some trade, craft, or profession (such as a "Master plumber", "Master carpenter" or "Master musician"). In this sense it evokes the guild organizations of the Middle Ages.