Anno Domini (A.D.) is a Latin phrase that means "in the year of the Lord". It is used to denote the years after the birth of Jesus Christ. The phrase was first used in the 6th century by Dionysius Exiguus, a Scythian monk who was commissioned by Pope John I to create a calendar that would be used to calculate the date of Easter. Dionysius based his calendar on the Julian calendar, which was the calendar used by the Roman Empire at the time. He chose the year 753 BC as the starting point for his calendar, which he believed was the year that Rome was founded. He then counted the years forward from 753 BC to the year 1 AD, which he believed was the year that Jesus Christ was born.
The Anno Domini calendar is still used today to calculate the date of Easter and other religious holidays. It is also used to date historical events. For example, the American Revolution began in 1775 AD.
There is another calendar era called "Before Christ" (B.C.) which is used to denote the years before the birth of Jesus Christ. For example, the Roman Empire began in 27 B.C.