quantum mechanics

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Quantum theory, especially the quantum theory of the structure and behavior of atoms and molecules.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The branch of physics which studies matter and energy at the level of atoms and other elementary particles, and substitutes probabilistic mechanisms for classical Newtonian ones.
• noun uncountable, by extension Something overly complicated or detailed.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the branch of quantum physics that accounts for matter at the atomic level; an extension of statistical mechanics based on quantum theory (especially the Pauli exclusion principle)