noun
Refers to a historical era or the starting point of a calendar system, such as 紀元前 (BC) or 紀元後 (AD). Often used in historical contexts.
この遺跡は紀元前3世紀のものだ。
These ruins date from the 3rd century BC.
キリスト紀元は西洋
The Christian era is the basis of the Western calendar.
noun
Common Era; AD
Specifically denotes the Common Era (CE) or Anno Domini (AD). Used in expressions like 紀元前 (BC) and 紀元後 (AD).
紀元前200年と紀元後200年では400年の差がある。
There is a 400-year difference between 200 BC and AD 200.