In ARIS context
The master object is the object to which objects are consolidated at definition level. This reduces the number of database items by consolidating all individual object relationships to other objects, models, etc., into the object definition of the master object in a way that is methodically consistent.
In Alfabet context
Database object for which a mapping is created in the other system.
If you create a mapping for an Alfabet object in ARIS, the Alfabet object is the master object.