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A piece of advertising. At first it was called advertisement, afterwards advert, and now just ad. They are very common in the internet. They can be displayed as popups, popunders, banners, skyscrapers, text ads and in many other ways.
Some companies exploit them as adware - programs that advertise on the computer they are installed on. Normally the user doesn't know that they're installed until seeing the first ads. Although, most people avoid using such over-advertised products because they have no interest in supporting such companies that exploit adware.
Some web browsers, like Firefox, have the ability to have add-ons installed. Such add-ons include ad blockers like Adblock Plus, that help the user remove unwanted flash, text, picture, and whole frame ads. For other browsers, users can install additional software that remove ads.
John: "Help me! I can't get rid of all those ads!"
Jake: "Don't worry! I know some good ad blockers."

Webmaster: "Man, since I use that adware you wrote my site has lost visitors!"
H4cK3R: "1 ' ) l3 3 7 7 3 ยฎ ยฎ 3 /\/\ 0 \/\/ 3 17 !"
by Some completely other guy August 17, 2007
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2
Anno Domini, Latin for "in the year of our lord". AD marks the years after the birth of Jesus Christ.

For politically correct reasons, AD is being replaced by CE.
The year right now is 2004 CE! I mean... AD... freakin' Christians and their weird year systems!
by Urban Dictionary October 28, 2004
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3
Short for the too short-lived show Arrested Development formerly on Fox.
Did you see the scene on AD last night when Tobias was dressed as a giant mole attacking a small model city and George Michael showed up with the jetpack? That was classic.
by tacostacos November 22, 2005
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A.D means Ass Disease like when a female has a big round ass.
damn those senoritas have a case of A.D
by AHDIU November 20, 2007
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actual day; actual date
"what was the a.d. of the purchase?"

"let's party on the a.d. of my b-day for a change!"
by kingman November 16, 2003
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All-Day.

I nickname given to Adrian Peterson of the Minnesota Vikings, because he'll run all over your team all day.
Did you see AD set the single game rushing record against the Chargers?
by TCT February 26, 2008
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