Questions about example sentences with, and the definition and usage of "Ad"

A: Ad lib presentation (Spontaneous speech or talk about an issue)

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A: I would probably say the sexond one in my audio message

A: 200 after descent

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A: assistant director

A: You should say, "When will the AD world cup start?" 😅

A: A company placed an advertisement for an available job in the newspaper and the person is calling because he/she is interested in interviewing.

A: It stands for Anno Domini, which is Latin for 'the year of our Lord'. Strictly speaking the initials should fall before the number- AD 1984, etc.
Our current calendar (in the English speaking world and much of the rest as well) is based on Christianity, specifically an early estimate for the birth of Christ which would begin the reckoning at the year AD 1. All of history prior to that event was renumbered backward from there and designated B.C. (before Christ). I have no idea why one is Latin and the other isn't.