What is the origin of the term "military power"?
The etiology of military power is from War Emergency Power (WEP) which in the WWII era was a higher than normal rating power (i.e. >100% rated power) setting on an aircraft engine.  Such power settings were approved for short durations (typically 5 minutes or less) such as takeoff and battle maneuvers.

The term was quickly shortened to military power.

Sometimes military power is assumed to be simply full throttle, but that may not be accurate, particularly in blown engines, where turbochargers or superchargers may be set to permit higher than normal air delivery to the combustion chamber.

Finally, military power was used for more extended periods of times, when conditions dictated, but at a reduction in reliability and service life.  Sometimes in war, service life is not a pilot's greatest concern.