What plasma is ? Plasma is by far the most common form of matter. Plasma in the stars and in the tenuous space between them makes up over 99% of the visible universe and perhaps most of that which is not visible. Plasma is partially or fully ionized gas comprising molecules, atoms, ions, electrons and photons. The concept of this state of matter was introduced by I. Langmuir and L. Tonks in 1929.
At temperatures below ~10 eV one refers to plasma as low-temperature or cold.  At T>10 eV full ionization of neutral particles occures and plasma is called high-temperature or hot.The main difference between plasma and gas is the presence of charged particles in plasma, i.e. electrons and ions what makes plasma electrically conductive.  Energy needed to strip electrons from atoms to make plasma can be of various origins: thermal, electrical, or light (ultraviolet light or intense visible light from a laser). With insufficient sustaining power, plasmas recombine into neutral gas.