The abbreviation "AD" stands for the Latin phrase "Anno Domini," which means "in the year of the Lord." It is used to count the years after the birth of Jesus Christ, starting with year 1 AD. This dating system was introduced by the Christian monk Dionysius Exiguus in the 6th century and later popularized by the English monk Bede the Venerable. The years before Christ's birth are labeled "BC," which stands for "Before Christ".
