Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in 1879 (he called it "radiant matter").
Plasma, in physics, fully ionized gas of low density, containing approximately equal numbers of positive and negative ions . It is electrically conductive and is affected by magnetic fields. The study of plasma, called plasma physics, is especially important in research efforts to produce a controlled thermonuclear reaction . Such a reaction requires extremely high temperatures; it has been computed that a temperature of about 10 million degrees Celsius would be needed to initiate the reaction between deuterium and tritium.
By passing a very high electric current through plasma great heat is produced and, simultaneously, an electromagnetic field is created, causing the plasma to withdraw from the walls of its container. The contraction of the plasma, called the pinch effect, prevents the container from being destroyed, but the effect may become unstable too quickly for the fusion reaction. The properties of plasma are distinct from those of the ordinary states of matter, and for this reason many scientists consider plasma a fourth state of matter. Interstellar gases, as well as the matter inside stars, are thought to be in the form of plasma, thus making plasma a common form of matter in the universe.
Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container; unlike gas, in the influence of a magnetic field, it may form structures such as filaments, beams and double layers. Some common plasmas are stars and neon signs.