AD was an era of time on Earth. (TNG: "The Big Goodbye", "The Royale", "Manhunt")
As late as the year 2079, it was still in effect. (TNG: "The Royale")
AD stands for "Anno domini", Latin for "In the year of our Lord".
In secular publications, like the Star Trek Encyclopedia (4th ed., vol. 1, p. 125), AD has been replaced by the dating reference system CE (Common Era).