基督纪年，又称为主历，是源自于基督教的纪年方法，以耶稣诞生年作为纪年的开始。在儒略历与格里高利历中，在耶稣诞生之后的纪年，称为主的年份（拉丁语：Anno Domini，缩写为A.D.；又译为主后），而在耶稣诞生之前的纪年称为主前（英语：Before Christ，缩写为B.C.）。现代学者为了淡化其宗教色彩，加上耶稣并非生于公元1年的谬误，多半改采用公元（Common Era，缩写为C.E.）与公元前（Before the Common Era，缩写为B.C.E.）的说法。

Before Christ redirects here, so this article deals with both "Anno Domini" and "Before Christ".

The terms anno Domini (AD or A.D.) and before Christ (BC or B.C.) are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin, which means in the year of the Lord but is often translated as in the year of our Lord. It is occasionally set out more fully as anno Domini nostri Iesu (or Jesu) Christi ("in the year of Our Lord Jesus Christ").