From AMS Glossary
An ionized gas composed of positive and negative charges (and possibly neutral atoms and molecules) of almost equal charge density.
At least one kind of charge is mobile. The term was coined by Langmuir and Tonks (1929) "to designate that portion of an arc-type discharge in which the densities of ions and electrons are high but substantially equal." A more quantitative definition can be given in terms of the Debye shielding distance, the distance over which the density of negative charges can be appreciably different from that of positive charges: A plasma is an ionized gas for which the Debye shielding distance is small compared with a characteristic length (Spitzer 1962). According to this definition the ionosphere is a plasma, and so is a slab of aluminum, but in atmospheric usage it is limited to an ionized gas.
Langmuir, I., and L. Tonks 1929. Phys. Rev.. 33. p. 196.
Spitzer, L. 1962. Physics of Fully Ionized Gases. 2d ed., . p. 22.