Plasma, in physics, a highly ionized gas, usually at high temperatures, that conducts electricity and is affected by a magnetic field. Plasma is sometimes called the fourth state of matter. Plasma is the most common form of matter in the universe; the sun and all other stars, as well as some types of interstellar matter, consist of plasmas. Plasmas occur in lightning bolts and in the regions of the earth's upper atmosphere called the ionosphere and the Van Allen radiation belts. Man-made plasmas occur in electric discharge lamps and in electric arcs used for welding.

A plasma is composed of equal numbers of positively charged ions (atoms that have lost one or more of their electrons) and free electrons, which have a negative electrical charge. The ions and electrons have a strong tendency to recombine and form electrically neutral atoms of ordinary gas.

Two factors can prevent a plasma from becoming an ordinary gas. One is a large supply of energy, usually in the form of heat, which can keep the ions and electrons moving too fast to recombine easily. The other is an extremely low density in the plasma. If the density is low enough, the space between the particles is so great that there is little chance for them to recombine.

One of the major branches of research on plasmas is directed toward the possibility of producing a sustained thermonuclear reaction in hydrogen plasma. However, in experiments conducted with laboratory equipment designed to produce high-temperature plasmas, temperatures and densities high enough to begin a nuclear fusion reaction have not yet been achieved.