A branch of physics dealing with physical properties of matter at temperatures such that thermal fluctuations are greatly reduced and effects of interactions at the quantum-mechanical level can be observed. As the temperature is lowered, order sets in (either in space or in motion), and quantum-mechanical phenomena can be observed on a macroscopic scale. As an example, consider a magnetic system. In a sample at high temperatures, there is a high degree of disorder corresponding to a large entropy because of the random orientations of its magnetic moments, the sample being in a paramagnetic state. At low temperatures, the sample is in a highly ordered state, the ferromagnetic state (lower entropy), and the details of the interactions responsible for this state can be studied.