Stands for "Personal Digital Assistant." These are the little electronic devices you see people jotting stuff down on in public. Usually, when you see someone with a PDA, they will be holding it out far front of them for everyone to see. Fortunately, as PDAs become more common, more people will have them and we won't have to deal with the people who make sure everyone else sees that they have one.
The first PDA, called the Newton, was created by Apple in 1993. Since then, numerous other companies have jumped on the bandwagon and have added many new designs and options to the PDA market. The Palm Pilot, Handspring Visor, HP Jordana, Compaq Aero, Sharp Mobilon, and Sony Cli? are all common PDAs. Ironically, Apple's Newton was discontinued when the company was having financial difficulties in 1998. Today's PDAs allow you to organize your schedule, take notes, do math calculations, play games, write memos, and even surf the Internet and send e-mail. They are cool things to have, but if you decide to get one, please do us all a favor and don't show it off in public.
PDA also stands for "Public Display of Affection," and though this term is all too relevant at college campuses, it has nothing to do with this definition.