Quantum theory
A theory which has the guiding principle that physical quantities can only have discrete values. It states that nature restricts the values which properties can have; this is known as quantitization. In the everyday world, the size of this quantitization is so small that it goes unnoticed. Only on the scale of sub-atomic particles is it noticeable. Apart from quantization the most familiar concepts of quantum theory are wave-particle duality, Heisenberg's Uncertainty Principle and Pauli's Exclusion Principle.
A branch of physics that describes the behavior of objects of atomic and subatomic size.