In synchronization this is the opposite of slave. In order for any two (or more) devices to sync to one another, one of them has to act as a master while others act as slaves and follow it. The master machine is, by definition, under its own control and dictates the location and timing or speed of playback. In modern applications it is often possible for a machine to be the master in one domain (such as location) while being a slave in another (such as speed). In many modern facilities all machines are driven by one master clock (speed) source and controlled by one master controller.
To master something is the act of preparing recorded material for mass duplication and is discussed in our WFTD Mastering.