Anno Domini (Medieval Latin: In the year of (the/Our) Lord), abbreviated as AD or A.D., is a designation used to number years in the Julian and Gregorian calendars. More fully, years may be also specified as Anno Domini Nostri Iesu (Jesu) Christi ("In the Year of Our Lord Jesus Christ").
The calendar era which it numbers is based on the traditionally reckoned year of the conception or birth of Jesus. Before Christ, abbreviated as BC or B.C., is used in the English language to denote years before the start of this epoch.
Though the Anno Domini dating system was devised in 525, it was not until the 8th century that the system began to be adopted in Western Europe. According to the Catholic Encyclopedia, even popes continued to date documents according to regnal years, and usage of AD only gradually became more common in Europe from the 11th to the 14th centuries. In 1422, Portugal became the last Western European country to adopt the Anno Domini system.
