From... http://en.wikipedia.org/wiki/Anno_Domini
Anno Domini (abbreviated as AD or A.D., sometimes found in the form Anno Domine) and Before Christ (abbreviated as BC or B.C.) are designations used to label years in the Julian and Gregorian calendars. The calendar era to which they refer is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, the central figure in Christianity, with AD denoting years after the start of this epoch, and BC denoting years before the start of this epoch. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. This dating system was devised in AD 525, but was not widely used until after AD 800.
Spain and Portugal continued to date by the Era of the Caesars or Spanish Era, which began counting from 38 BC, well into the Middle Ages. In 1422, Portugal became the last Catholic country to adopt the Anno Domini system.
From... http://wiki.answers.com/Q/What_is_the_english_translation_of_the_latin_anno_domini#ixzz1A4zKN400
Domini (Imperial Rome) - meaning "lords" in Latin, a title of emperors of the Roman Empire.