A "Fourth State of Matter" in which many of the atoms or molecules are ionized. Plasmas have unique physics compared to solids, liquids, and gases. (Most plasmas can be thought of at first as extremely hot gases, but their properties are generally quite different.) Some (but not all!) Examples include the sun, fluorescent light bulbs and other gas-discharge tubes, very hot flames, much of interplanetary, interstellar, and intergalactic space, the earth's ionosphere, parts of the atmosphere around lightning discharges, and of course the plasmas in fusion reactors and other plasma physics experiments. Types of plasmas include - Astrophysical, Collisionless, Cylindrical, Electrostatically Neutral, Inhomogeneous, Intergalactic, Interstellar, Magnetized, Nonthermal, Partially Ionized, Relativistic, Solid State, Strongly Coupled, Thermal, Unmagnetized, Vlasov and more.