Plasma
Plasma is a state of matter in which a gas is so energized that electrons are stripped from atoms, forming a conducting mix of ions and free electrons.
Definition
Plasma is a super-hot, glowing state of matter whose atoms have been stripped of electrons, producing a mixture of positive ions and free electrons, and it is considered the fourth state of matter. Unlike an ordinary gas, plasma conducts electricity and responds to magnetic fields, and it typically glows as electrons recombine with ions after being torn away by very high temperatures or strong electric fields. Formally, plasma is a quasi-neutral collection of ions and free electrons whose collective behavior is governed by long-range electromagnetic forces rather than binary collisions, characterized by its degree of ionization, electron temperature, and density, and capable of phenomena absent in neutral gases such as Debye shielding, plasma oscillations, and magnetic confinement.
Example
The sun is a giant ball of plasma, lightning is plasma, and the glowing gas inside a neon sign is plasma too; in a fluorescent light, an electric current turns mercury vapor into plasma that emits ultraviolet light, which the tube's white coating converts into visible light, while the sun's core is hydrogen plasma at about $15$ million kelvin. In a tokamak, deuterium-tritium plasma is heated above $100$ million kelvin, several times hotter than the sun's core, so its nuclei collide with enough energy to fuse, and magnetic fields of several tesla confine it because no physical wall could survive touching it.
Key Insight
Plasma is rare on Earth but the most common state of matter in the universe, since stars are made of it, and it is not simply a hotter gas: its free charges let it carry current and be shaped by magnets, which is exactly how fusion reactors keep plasma away from their walls. Plasma also sits between chemistry and physics, since the same emission lines chemists use to identify elements in a flame test come from plasma physics, and astronomers read a star's composition directly from the light its plasma emits.