Any ionized gas cannot be called plasma, of course; there is always some small degree of ionization in any gas. A useful definition is as follows: A plasma is a quasi neutral gas of charged and neutral particles which exhibits collective behavior.
Consider the forces acting on a molecule of, say ordinary air. Since the molecule is neutral, there is no net electromagnetic force on it, and the force of gravity is negligible. The molecule moves undisturbed until it makes a collision with another molecule, and these collisions control the particle’s motion. A macroscopic force applied to a neutral gas, such as from a loudspeaker generating sound-waves, is transmitted to the individual atoms by collisions. The situation is totally different in a plasma, which has charged particles. As these charges move around, they can generate local concentrations of positive or negative charge, which give rise to electrical fields. These fields affect the motion of other charged particles far away.
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