The term Don (don, roughly 'Lord') abbreviated as D., is an honorific prefix primarily used in Spain and Hispanic America, and with different connotations also in Italy, Portugal and its former colonies, Croatia, and formerly in the Philippines.
Don is derived from the Latin dominus: a master of a household, a title with background from the Roman Republic in classical antiquity. With the abbreviated form having emerged as such in the Middle Ages, traditionally it is reserved for Catholic clergy and nobles, in addition to certain educational authorities and persons of distinction. Dom is the variant used in Portuguese.
In Britain and Ireland, especially at Oxford, Cambridge, and Dublin, the word is used for a college fellow or tutor, but it is not used as an honorific prefix.
In Spanish, 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, rather than in place of, a person's name.
Syntactically, in Spanish, don and doña are used in a way similar to "mister" (señor) and "missus" (señora), but convey a higher degree of reverence, although not necessarily as high as knightly or noble titles such as lord and dame. Unlike The Honourable in English (but like the English Sir for a knight or baronet), 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 simply "Don Diego" (the secret identity of Zorro) are typical forms. But a form using the last name (e.g. "Don de la Vega") is not considered correct and rarely would be used by Spanish speakers ("señor de la Vega" would be used instead).
The feminine equivalents are Doña (ˈdoɲa), Donna (ˈdɔnna), Doamnă (Romanian) and Dona (ˈdonɐ) abbreviated 'D.a', 'Da.', or simply 'D.