A wonder weapon is any very advanced form of weaponry whose design, production or deployment in the field of battle is impractical or ill-timed. The term is a calque of the German word Wunderwaffen, which was coined during World War II. Amongst Germans of that era, it was abbreviated into "Wuwa" /vu va/, which belittled the projects.
Such weapons are radical departures from, or improvements upon, the conventional arms of the time and are intended to turn the tide of a current conflict. These weapons are either never completed or are introduced too near the end of hostilities and in such limited numbers that the intended effect never meets expectations.
The secrecy surrounding the development of new weapons sometimes conflicts with the weapon developers’ hype of new products, resulting in common or 'evolutionary' advancements being mislabelled as wonder weapons. The term 'wonder weapon' can also be used intentionally to have a moral-boosting effect for propaganda purposes.
The term is most often used to describe weapons that were being developed in Nazi Germany just prior to the end of the war, such as the Wasserfall missile, the Messerschmitt Me 262 (the world's first jet fighter) and the failed German atomic bomb project led by Werner Heisenberg. It may also refer to the V-weapons program.
Historically, wonder weapons of the past have become the conventional weapons of later conflicts when fully developed--this happened to dreadnoughts, tanks, and long-range missiles. More recently, offensive developments in electromagnetism with railguns, lasers, and microwave or radio frequency radiation could be considered wonder weapons.