Short answers, pulled from the story.
Plasma is a state of matter that results when one of the other three states, often a gas, undergoes an appreciable degree of ionization. It consists of a significant portion of charged particles, meaning ions and electrons, which make it electrically conductive. It is called the fourth state of matter after solid, liquid, and gas.
Plasma is called the fourth state of matter because it follows solid, liquid, and gas, and behaves distinctly from all of them. Sir William Crookes presented the idea of a fourth state of matter in a well-known paper in 1879. Unlike the transitions between the other three states, the change to plasma is not well defined.
Irving Langmuir introduced the term plasma to describe ionized gas in 1928. According to Lewi Tonks and Harold Mott-Smith, who worked with him in the 1920s, Langmuir chose the word by analogy with blood plasma, because electrons traveling from thermionic filaments reminded him of how blood plasma carries corpuscles and germs.
An estimated 99.9% of all ordinary matter in the universe is plasma, and it makes up more than 99% of the matter observed in the visible universe. Stars are almost pure balls of plasma, and plasma dominates the rarefied intracluster and intergalactic medium. On Earth it is rarely encountered.
Terrestrial plasmas include lightning, the polar aurorae, the ionosphere, and St. Elmo's fire. Neon signs and lightning are examples of partially ionized plasma. Artificially produced plasmas include plasma displays, fluorescent lamps, neon signs, rocket exhaust, and ion thrusters.
Most artificial plasma is generated by applying electric or magnetic fields through a gas, with energy input required to produce and sustain it. In a discharge tube, rising voltage stresses a dielectric gas beyond its dielectric strength into electrical breakdown, marked by a spark. The Townsend avalanche then multiplies charged particles into the millions after about 20 successive sets of collisions.
Plasma is used in extractive metallurgy, surface treatments such as plasma spraying, etching in microelectronics, metal cutting and welding, vehicle exhaust cleanup, and fluorescent lamps. Arc discharge near 10,000 K is used to smelt minerals containing Al2O3 to produce aluminium. The dielectric barrier discharge led to a field known as plasma medicine after it was shown to inactivate bacterial cells in the mid-1990s.