Anno Domini Nostri Iesu Christi ("In the Year of Our Lord Jesus Christ"), commonly shortened to Anno Domini ("In the Year of the Lord"), abbreviated as AD or A.D., is the designation used to number years in the Christian Era, conventionally used with the Julian and Gregorian calendars. It defines an epoch based on the traditionally-reckoned year of the birth of Jesus Christ. BC (Before Christ) is used to denote years before Anno Domini years in English.
Breakdown of Christian Era Terminology
AD or (A.D.) (anno Domini) — the Latin phrase meaning in the year of our Lord Jesus Christ, the abbreviation AD is formally placed before a year of the Christian era (i.e. AD 1999), but can also be placed after a year (i.e. 2006 AD).
BC or (B.C.) (before Christ) — the abbreviation BC is placed after a year that precedes the Christian era (i.e. 90 BC). Original Latin term "Ante Christum Natum" ( aCn) is not usually preferred in the English language.
CE or (C.E.) (common era) — the religion-neutral alternative to Anno Domini placed after a year of the Christian/common era (i.e. 2001 CE). See Wikipedia article on Common Era for more details.
BCE or (B.C.E.) (before common era) — the religion-neutral alternative to Before Christ. This term, created in conjunction with the Common Era (CE) designation, is formally placed after a years prior to the Christian/current era (i.e. 400 BCE).