QM - Quantum Mechanics has come to mean the study of the nature of matter at extremely small scales: at, or smaller than the atom, or sub-atomic particle scale.

Quantum Mechanics studies matter at the level where gradient effects seen in macro-scale observations no longer exist. That is, gradient effects, which are caused (presumably) by the collective effects of many quantum-scale events, can't be observed at the QM scale.

At this scale, changes do not continuously move from one state to another, instead they simply jump between the two states, with no intervening "movement" states. This is the concept underlying the name (Quantum), as well as the catch-phrase: "Quantum Leap".

Important: Though this is about our inability to measure, it is also more fundamental than that. Consider the following prediction: