Domĭnus
A master, owner. The word was applied as a title of respect to a superior in rank or station. Thus the head of a family was sometimes called dominus by the free members of his family as well as by his slaves. The title of dominus came to be ascribed to the emperor. The history of this use of the word, which corresponds with changes in the character of the emperor, is briefly as follows: Augustus refused to be called dominus (Suet. Aug. 53, with Peck's note), as did also Tiberius (Suet. Tib. 27); and Caligula was the first emperor who allowed himself to be addressed by the title ( Victor. Caes. 3). Domitian claimed the titles of Deus et Dominus (Dio Cass. lxii. 13). Trajan only wished to be called princeps (Paneg. 2, 63, 88). Pliny in his letters always addresses Trajan as dominus, but in doing so he does not intend to make use of an official title. Dominus first appears on imperial monuments in the reign of Septimius Severus. Aurelian first adopted the title Deus et Dominus on his coins. Diocletian allowed himself to be publicly addressed as dominus. From the fourth century the emperors freely ascribe the title to themselves. The reason why the earlier emperors objected to being styled dominus is to be found in the fact that they still kept up a pretence of republican equality. Now the word dominus to a Roman, like δεσπότης to a Greek, means a master in relation to slaves, or (politically) a tyrant, the possessor of arbitrary power (Sall. Iug. 85). Later, when the imperial power had become hedged about by precedent and tradition, the emperor willingly accepted the title as his due.The word dominus, besides retaining its full force as a term of great dignity, underwent a further development as a social title. In Suet. Claud. 21, it is applied by the emperor to his plebeian guests as an ordinary title of courtesy. In Mart. vi. 88, it is equivalent to “Mister.” It is used by Ovid ( Am. iii. 7.11) as a term of affection. In the modern languages it has developed through the Low Latin forms domnus, domna, donnus, dominicella into the Portuguese dom, Span. don, doña, Fr. dame, madame (mea domna), demoiselle, and Old English dan. See Peck's note to Suet. Aug. 53.