Master is a commonly used name throughout the Imperium. There are several different definitions used, some of which are below:
Space Marines
Master in this case refurs to the Chapter Master of the Space Marine and is the stronger Commander unit available to the Space Marines, the other being Captain1.
Administratum
Masters are the heads of department of the Administratum, in charge of an entire office or division. Most masters also have a special unique title to their division, such as the Chancellor of the Estrate Imperium and the Historicus of the Historical Revision Unit2.