Any suit of armor which gives mechanical aid to the wearer. Typically, the wearer will use natural motions for walking or other actions. The suit instantly detects and duplicates whatever action the wearer takes. Additional capabilities can be built in, such as gyroscopic stabilization (this would make balancing much easier), or even the ability to fly with jump jets.
Power Armor is a concept frequently found in Science Fiction. As far as I can tell, the first instance of Power Armor was in Starship Troopers (the book, not the movie), though it was not called that. Very few features have been thought up since then. Aside from the individual planetary atmospheric entry capability, each marine had limited flight; a wide assortment of weaponry, from talking time bombs designed to minimize civilian casualties, to mini-nukes; a radio link to the rest of the squad; heads-up display of tactical information; and food and drink dispensers (and a urinary catheter).
Since then, the concept has blanketed science fiction of all forms (especially military science fiction), from the written word (too many to count, but including The Forever War, Redliners, Hyperion, The Honor of the Queen, and Hydrogen Guy), to RPG's (Fallout, Rifts, Robotech, Mechwarrior, and even FF6), to first person shooters (Mechwarrior again), to tactical simulators (in Starcraft, even the lowly marines are officially wearing powered armor, though the Goliaths look a lot more convincing), to turn-based strategy games (Masters of Orion II), with a few appearances in movies (a minor appearance in Aliens, and strangely enough, NOT Starship Troopers). And of course, all mecha (e.g. Evangelion, Mobile Suit Gundam) would count as power armor.
While power armor is not a fully realized technology in reality, the DoD was looking for development proposals in 2001. The project requirements did not include the most common trait of science fiction powered armor - perfect protection from small arms fire - but that will probably be added after the basic requirements have been solved:
- Must be able to support its own weight and carry significant additional equipment
- Must be able to operate for extended periods (it then defined extended periods)
- Must provide power and space for an independent onboard computer.
- Must not become a walking bomb (i.e. must protect the power systems)
Then there were some budget restrictions and so forth. I do not know whether there were any takers, but I suspect there were. The main problem at this point is the ability to operate for extended periods. Efficient energy storage in a rapidly accessible form is not easy. | 
