In the vernacular, the term quantum leap has come to mean an abrupt change or "step change", especially an advance or augmentation. The term dates back to early-to-mid-20th century, coinciding with the discoveries of quantum mechanics. The popular and scientific terms are similar in that both describe a change that happens all at once (revolutionary), rather than gradually over time (evolutionary), but the two uses are different when it comes to the magnitude of the change or advance in question.
In real physical systems a quantum leap is not necessarily a large change, and can in fact be very insignificant. A good example of this can be taken from the Bohr model of the hydrogen atom, where the observed energy shifts associated with shifts of different quantum states (quantum leaps) span a wide range from large to small (when compared to the energy required to completely free an electron). In the popular sense, the term is usually applied to mean a large or significant change, which is thus not strictly correct.