Historians use terms such as a. d. or b. c., decade, and century to?

To be specific when identifying a certain time within our History. A.D., "Ano Domini" (i think thats how its spelle marks the years after the death of Christ, and B.C. means "Before Christ". These are used to help distinguish how long ago they were. The terms, such as "decades" and "centurys" are used to be shorter. So if you were writing a paper, instead of saying "545 years ago..." or if you dont know the exact years, you can simply put "centurys ago...".

hope this helps!!