The full form of AD is Anno Domini. Anno Domini is Medieval Latin and means "in the year of the Lord". The purpose of the B.C./A.D. dating system was to make the birth of Jesus Christ the dividing point of world history. AD counts years from the start of this epoch, and BC denotes years before the start of the era.
Art Director (AD) is the title for a variety of similar job functions in creative organizations like graphic design, film, music, fashion, advertising, media or entertainment industries etc
Advertisement (Ad) is a paid announcement in a public medium (newspaper, radio, internet) promoting a product, service or event.
The Assistant Director (AD) is the director’s right hand person, taking responsibility for a number of important practicalities like tracking daily progress against the filming production schedule, arranging logistics, preparing daily call sheets, checking cast and crew, maintaining order on the set etc. so that the director is free to concentrate on the creative process.