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Dominus is the Latin word for master or owner. As a title of sovereignty the term under the Roman Republic had all the associations of the Greek Tyrannos; refused during the early principate, it finally became an official title of the Roman Emperors under Diocletian (this is where the term dominate, used to describe a political system of Roman Empire in 284-476, is derived from). Dominus, the French equivalent being "sieur", was the Latin title of the feudal, superior and mesne, lords, and also an ecclesiastical and academical title. The ecclesiastical title was rendered in English "sir", which was a common prefix before the Reformation for parsons, as in Sir Hugh Evans in Shakespeare's Merry Wives of Windsor. In the past, the academical use was for a Bachelor of Arts. The shortened form "Dom" is used as a prefix of honor for ecclesiastics of the Catholic Church, and especially for members of the benedictine and other religious orders.
For women, a Domina, in old English Law, was a title formerly given to noble ladies who held a barony in their own right. At the University of Cambridge, the honorific 'Domina' (abbreviated as Dna) is given to women who hold a Bachelor of Arts degree, but not a master's degree. From mea domina, "my lady," through French "madame," comes "madam", and the contracted form "ma'am."
Evolvement into Don/Dom
The honorific Dom, and its feminine form Dona (both abbreviated as D.) is also a title of honor in Portugal, as formerly in Brazil, used by members of the blood royal and others on whom it has been conferred by the sovereign.
The Spanish form "Don" is also a title, formerly applicable only to the nobility, and now one of courtesy and respect applied to any elderly man. The feminine form Doña is similarly applied to a lady.
In French, the words Dame and Madame (respectively Lady and Mylady), derived from Latin Domina, are still the normal way to address a woman.