It has been explained to me as the amount of power used in a plane to maintain a specific altitude, airspeed or possibly, some combination of both.
Something like using different combinations of throttle, prop pitch, rpm, etc to effect the same rate of climb or airspeed.
Good answers guys, but I found the easiest way to explain it to a student pilot is this way: If you get the aircraft in an attitude where you have to trade altitude for airspeed, because you are to slow and the engine will not develop enough power to maintain altitude and accelerate to a safe flying speed, then you are indeed, behind the power curve. Easiest aircraft in Aces High to get behind the power curve, the B-29, or any of the four engine bombers in the game.
Best thing for any pilot in real life to do is know his POH from front to back, with particular attention to the VSO speed in different configurations and add 10 knots and you will never get in trouble. We used to have a saying when shooting approaches in icing conditions, VREF plus 10 knots and 10 more knots for wife and kids! Of course in heavy multi-engine aircraft such as the B-29, you knew what your approx. weight on landing would be, so you had a "bug" speed, much like the airline jets of today have.
