dominus, in ancient Rome, “master,” or “owner,” particularly of slaves. The name became the official title for the emperor, beginning with Diocletian, who reigned from ad 284 to 305; and thus he and his successors are often referred to as the dominate (dominatus), as contrasted with the earlier principate (principatus) of Augustus and his successors. Some earlier emperors, such as Caligula (reigned ad 37–41), however, also had used the title. By Trajan’s day it was the common form of address to the emperor.
In the Latin church, Dominus was used as the equivalent of the Hebrew Adonai and the Greek Kyrios, to refer to the Christian God. Dominus in medieval Latin referred to the “lord” of a territory or the overlord of a vassal. It was later used as a respectful form of address (Spanish don, Portuguese dom) and for the clergy (Italian don).