Merriam-Webster Collegiate Dictionary - quantum mechanics

 noun plural but singular or plural in construction  Date: 1922 a theory of matter that is based on the concept of the possession of wave properties by elementary particles, that affords a mathematical interpretation of the structure and interactions of matter on the basis of these properties, and that incorporates within it quantum theory and the uncertainty principle — called also wave mechanics  • quantum mechanical adjective  • quantum mechanically adverb