Uncertainty principle
The expressions "quantum leap" and "quantum jump" have become ordinary ways of talking about things. Usually people intend to describe something as involving a huge change that occurs over a short period of time. The term actually applies to the way an electron behaves in an atom either when it absorbs a photon coming in from the outside and so jumps from one orbit around the atom's nucleus to a higher orbit, or when it emits a photon and so falls from a higher orbit to a lower orbit. The idea of Niels Bohr and his colleagues was that the electron does not move between orbits but instead it disappears from one orbit and instantaneously appears in another orbit. So a quantum jump is really not some earth shattering change, but a sudden tiny change.