- Glossary definition: Anno Domini
Anno Domini
(Medieval Latin: In the year of the Lord)

Anno Domini (AD or A.D.) and Before Christ (BC or B.C.) are designations used to label or number years used with the Julian and Gregorian calendars. System was devised in 525 by Dionysius Exiguus. Previously, there were two common ways of referring to dates. One was to use indictions, which were 15-year cycles, counting from 312 AD. There were three different varieties of indiction, each starting on a different day of the year. The other approach was to use regnal years—the reigning Roman emperor, for example, or the ruler of whichever kingdom was under discussion. This meant that in discussing conflicts between kingdoms, the date would have to be given in the regnal years of all the kings involved.