Don (Spanish: [ˈdon], Italian: [ˈdɔn], Portuguese: Dom [ˈdõ]) from Latin dominus, is an honorific title used in Iberia and Italy. The female equivalent is doña (Spanish: [ˈdoɲa]), Donna (Italian: [ˈdɔnna]), and Dona (Portuguese: [ˈdonɐ]), abbreviated "Dª" or simply "D."
Although originally a title reserved for royalty, select nobles, and church hierarchs, it is now often used as a mark of esteem for a person of personal, social or official distinction, such as a community leader of long standing, a person of significant wealth, or a noble, but may also be used ironically. As a style, rather than a title or rank, it is used with, and not instead of, a person's name.
Syntactically, it is used in much the same way (although for a broader group of persons) as "Sir" and "Dame" are used in English when speaking of or to a person who has been knighted, e.g. "Don Firstname" or "Doña Firstname Lastname". Unlike "The Honourable" in English, Don may be used when speaking directly to a person, and unlike "Mister" it must be used with a given name. For example, "Don Diego de la Vega," or (abbreviating "señor") "Sr. Don Diego de la Vega," or simply "Don Diego" (the secret identity of Zorro) are typical forms. But a form like "Don de la Vega," is not correct.
In North America, Don has also been made popular by films depicting the Mafia, such as The Godfather series, where the crime boss would claim for himself the signs of respect that were traditionally granted in Italy to nobility. This usage of the honorific in these films (e.g. Don Corleone, Don Barzini, etc.) is not common or correct in normal historic usage in Italy. The proper Italian usage is similar to the Castilian Spanish usage mentioned in the previous paragraph.
Spain and the Spanish Colonies 
Historically, don was used to address members of the nobility, e.g. hidalgos and fidalgos, as well as members of the secular clergy. The treatment gradually came to be reserved for persons of the blood royal, and those of such acknowledged high or ancient aristocratic birth as to be noble de Juro e Herdade, that is, "by right and heredity" rather than by the king's grace. But by the twentieth century it was no longer restricted in use even to the upper classes, since persons of means or education (at least of a "bachiller" level -equivalent of a Bachelor-), regardless of background, came to be so addressed and, it is now often used as if it were a more formal version of Señor, a term which was also once used to address someone with the quality of nobility (not necessarily holding a nobiliary title). This was, for example, the case of military leaders addressing Spanish troops as "señores soldados" (gentlemen-soldiers). In Spanish-speaking Latin America, this honorific is usually used with people of older age.
In Spanish colonial Philippines, this honorific address was reserved to the local nobilities - the Principalía,(p218) whose right to rule was recognized by Philip II on 11 June 1594.(tit. VII, ley xvi)
The honorific title Don is widely used in Latin America. This is the case of the Mexican New Age author Don Miguel Ángel Ruiz, the Chilean television personality Don Francisco, and the Puerto Rican industrialist and politician Don Luis Ferre, among many other figures. The title Don is considered highly honorific, more so than, for example, academic titles such as "Doctor" or than political titles such as "Governor." For example, although Puerto Rican politician Pedro Albizu Campos had a doctoral degree, he has been titled Don. Likewise, Puerto Rican Governor Luis Muñoz Marin has oftentimes been called Don Luís Muñoz Marin instead of Governor Muñoz Marin. In the same manner, Don Miguel Ángel Ruiz is an M.D.
Prior to the American conquest of the Southwest, a number of Americans immigrated to California, where they often became Mexican citizens and changed their given names to Spanish equivalents, for example "Juan Temple" for Jonathan Temple. It was common for them to assume the honorific "don" once they had attained a significant degree of distinction in the community.
Today in Mexico and in Mexican-American communities, the title Don or Doña is used in honorific form when addressing a senior citizen.
Portugal and Brazil 
The usage of Dom was a prerogative of princes of the royal blood and also of other individuals to whom it had been granted by the sovereign. In most cases, the title was passed on through the male line. Strictly speaking, only females born of a nobleman bearing the title Dom would be addressed as Dona, but the style was not heritable through daughters. The few exceptions depended solely on the conditions upon which the title itself had been granted. A well-known exception is the descent of Dom Vasco da Gama.
There were many cases, both in Portugal and Brazil, in which the title of Dom (or Dona) was conceded to, and even bought by, people who were not from the royalty. In any case, when the title was officially recognized by the proper authority, it became part of the name.
Today, in Portugal and Brazil, Dom is ordinarily employed only for higher members of the clergy, and for superiors of religious orders, such as the Order of Saint Benedict, wherein it is also associated with the status of Dom Frater. Dom is similarly used within the Benedictine Order throughout France and the English speaking world. In France, it is also used within the male branch of the Carthusian Order.
In the Portuguese language, the female, Dona (or, more politely, Senhora Dona), has become common when referring to a woman who does not hold an academic title. It's commonly used to refer to First Ladies, although less common for female politicians.
Officially, Don was the style for a principe or duca (and any legitimate, male-line descendant) who was a member of the nobility (as distinct from a reigning prince or duke, who was generally entitled to some form of the higher style of Altezza). This was how the style was used in the Almanach de Gotha for extant families in its third section. The feminine, "Donna", was borne by their wives and daughters. Genealogical databases and dynastic works still reserve the title for this class of noble by tradition, although it is no longer a right under Italian law.
In practice, however, the style Don/Donna (or Latin Dominus/Domina) was used more loosely in church, civil and notarial records. The honorific was often accorded to the untitled gentry (e.g., knights or younger sons of noblemen), priests, or other people of distinction. It was, over time, adopted by organized criminal societies in Southern Italy (including Naples, Sicily, and Calabria) to refer to members who held considerable sway within their hierarchies.
Today in Italy, the title is widely given everywhere only to Diocesan Catholic priests, (never for prelates, who bear higher honorifics such as monsignore, eminenza and so on). Outside of the priesthood or old nobility, usage is now fairly uncommon in the south and rarely if ever used in central or northern Italy. It can be used satirically or ironically to lampoon a person's sense of self-importance.
As in the Spanish usage, Don is prefixed either to the full name or to the person's given name, less commonly to the surname alone (as is the custom of the heads of mafia syndicates). The feminine Donna (with capital initial) is rarely used nowadays.
Other uses 
At Oxbridge, a member of the academic staff is sometimes referred to as a don—a remnant of the time when the universities were considered religious institutions and their staff a kind of clergy. In practice it is most commonly used to refer to fellows of the colleges. For example, "Don's Diary" in Cambridge Alumni Magazine.
- For more information about the social system of the Indigenous Philippine society before the Spanish colonization confer Barangay in Enciclopedia Universal Ilustrada Europea-Americana, Madrid: Espasa-Calpe, S. A., 1991, Vol. VII, p.624.
- BLAIR, Emma Helen & ROBERTSON, James Alexander, eds. (1906). The Philippine Islands, 1493–1898. Volume 40 of 55 (1690–1691). Historical introduction and additional notes by Edward Gaylord BOURNE;. Cleveland, Ohio: Arthur H. Clark Company. ISBN 978-0559361821. OCLC 769945730. "Explorations by early navigators, descriptions of the islands and their peoples, their history and records of the catholic missions, as related in contemporaneous books and manuscripts, showing the political, economic, commercial and religious conditions of those islands from their earliest relations with European nations to the close of the nineteenth century."
- de León Pinelo, Antonio Rodríguez & de Solórzano Pereira, Juan, eds. (1680). Recopilación de Leyes de los Reynos de las Indias (pdf) (in Spanish). Libro Sexto. " Títulos: i De los Indios. ii De la libertad de los Indios. iii De las Reducciones, y Pueblos de Indios. iv De las caxas de censos, y bienes de Comunidad, y su administracion. v De los tributos, y tassas de los Indios. vi De los Protectores de Indios. vii De los Caciques. viii De los repastimientos, encomiendas, y pensiones de Indios, y calidades de los titulos. ix De los Encomenderos de Indios. x De el buen tratamiento de los Indios. xi De la sucession de encomiendas, entretenimientos, y ayudas de costa. xii Del servicio personal. xiii Del servicio en chacras, viñas, olivares, obrajes, ingenios, perlas, tambos, requas, carreterias, casas, ganados, y bogas. xiv Del servicio en coca, y añir. xv Del servicio en minas. xvi De los Indios de Chile. xvii De los Indios de Tucuman, Paraguay, y Rio de la Plata. xviii De los Sangleyes. xix De las confirmaciones de encomiendas, pensiones, rentas, y situaciones."
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