Wikipedia
Wunderwaffe is German for "Wonder Weapon" and was a term assigned during World War II by the Nazi Germany propaganda ministry to a few revolutionary "superweapons". Most of these weapons however remained prototypes, which either never reached the combat theater, or if they did, then too late or in too insignificant numbers to have a military effect.
The V-weapons, which were developed earlier and saw considerable deployment, especially against London and Antwerp, trace back to the same pool of highly inventive armament concepts. Therefore, they are also included here.
As the war situation worsened for Germany from 1942, claims about the development of revolutionary new weapons which could turn the tide became an increasingly prominent part of the propaganda directed at Germans by their government. In reality, the advanced weapons under development generally required lengthy periods of design work and testing, and there was no realistic prospect of the German military being able to field them before the end of the war. When some advanced designs, such as the Panther tank and Type XXI submarine, were rushed into production their performance proved disappointing to the German military and leadership due to inadequate pre-production testing or poorly planned construction processes.
In the German language the term Wunderwaffe generally refers to a universal solution which solves all problems related to a particular issue, mostly used ironically for its illusionary nature.