It's a category of consumer electronics that includes any handheld computer device that is an electronic version of a personal organizer. It usually includes a calendar, address book, calculator, and a memo pad.
The term "personal digital assistant" was coined on January 7, 1992 by then Apple Computer CEO John Sculley at the Consumer Electronics Show in Las Vegas, Nevada, referring to the Apple Newton. Earlier devices like the Psion and Sharp Wizard already had the functionality to be considered PDAs, however. PDAs are some times referred to as "Palms" or "Palm Pilot" after an early PDA created by Palm, Inc. This usage is a case of genericized trademark, similar to referring to a tissue as a "Kleenex" or a bandage as a "Band-Aid".
PDA also stands for "Public Displays of Affection", but the first definition is definitely more interesting. ;-)
Source(s):
Wikipedia.com