"A quantum leap"
Hmm. I guess SelfAdjoint's explanation is as good as any. The way I heard it was that back in the 19th century there were a lot of problems with classical physics that nobody had been able to explain. Then within just a few short years you had a whole bunch of young scientists, Planck, Bohr, Einstein, Heisenburg, Pauli, etc. completely revolutionise the science of physics. I.e. a quantum leap is a profound leap forward in a very short time. In addition to it's more literal quantum mechanical definitions.