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. The academical use was for a Bachelor of Arts, and so is still used at the University of Cambridge and other universities. 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.
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.
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 member of the better classes. The feminine form Doña is similarly applied to a lady.
The English colloquial use of "don" for a fellow or tutor of a college at a university is derived either from an application of the Spanish title to one having authority or position, or from the academical use of dominus. The earliest use of the word in this sense appears, according to the New English Dictionary, in Souths Sermons (1660). An English corruption, "dan", was in early use as a title of respect, equivalent to master. The particular literary application to poets is due to Edmund Spenser's use of Dan Chaucer, well of English undefiled.