quantization The fact that light and matter on small scales behave in a discontinuous manner, and manifest themselves in the form of tiny "packets" of energy, called quanta.
quantum A discrete physical unit into which something can be divided, for example a photon, or the difference in energy states in an atom.
quantum mechanics The framework of laws governing the behavior of particles on atomic and subatomic levels. It starts with the assumption that energy is not infinitely divisible, but comes in quanta.
quark A fractionally charged, basic building block of protons, neutrons, and other elementary particles.
quasars Originally, a distant, highly luminous object that looks like a star. Strong evidence now exists that a quasar is produced by gas falling into a supermassive black hole in the center of a galaxy. [More Info: Field Guide]
Quasi-periodic Oscillations (QPO's) Variations in the intensity of X-radiation from sources that show periodic behavior for short time intervals, and a variety of periods.