Quantum mechanics is a fundamental physical theory which extends and corrects Newtonian mechanics, especially at the atomic and subatomic levels. The term quantum (Latin for how much) refers to the discrete units that the theory assigns to certain physical quantities, such as the energy of an atom at rest (see Figure 1, at right).

Quantum mechanics is a theory of mechanics, a branch of physics that deals with the motion of bodies and associated physical quantities such as energy and momentum. Mechanics can be subdivided into classical mechanics, relativistic mechanics, non-relativistic quantum mechanics, and relativistic quantum mechanics (quantum field theory). Quantum mechanics is regarded as more "fundamental" than the first two theories of mechanics, because the predictions of quantum mechanics have never been disproven after a century's worth of experiments. We will use the term "quantum mechanics" to refer to both relativistic and non-relativistic quantum mechanics; the terms quantum physics and quantum theory are synonymous. It should be noted, however, that certain authors refer to "quantum mechanics" in the more restricted sense of non-relativistic quantum mechanics.

Quantum mechanics is the underlying framework of many fields of physics and chemistry, including condensed matter physics, quantum chemistry, and particle physics. It describes with great accuracy and precision many phenomena where classical mechanics drastically fails, including the behavior of systems at atomic length scales and below (for instance, classical mechanics is unable to account for the existence of stable atoms), as well as special macroscopic systems such as superconductors and superfluids. However, quantum mechanics also reduces to classical mechanics in some physical situations; this property of quantum mechanics was highlighted by Niels Bohr, and is known as the correspondence principle. Most physicists now believe that quantum mechanics is a correct theory of the physical world (but not near black holes or when considering the observable Universe as a whole - the domain of general relativity). The question of compatibility between quantum mechanics and general relativity remains an area of active research.

Quantum mechanics predicts at least three classes of phenomena that classical mechanics and classical electrodynamics cannot account for: (i) the quantization (discretization) of certain physical quantities, (ii) wave-particle duality, and (iii) quantum entanglement.

The foundations of quantum mechanics were established during the first half of the 20th century by Max Planck, Albert Einstein, Niels Bohr, Werner Heisenberg, Erwin Schrodinger, Max Born, John von Neumann, Paul Dirac, Wolfgang Pauli and others. Some fundamental aspects of the theory are still actively studied.