Plasma density is a way of referring to the degree of ionization of plasma. Plasma is generated when enough energy is added to gas particles so that they start ionizing : atoms of the gas start losing electrons. Ionization changes the physical properties of the substance and it becomes plasma. Plasma density is the measure of what percentage of gas particles have ionized; more specifically it is the measure of how many free electrons there are per unit of volume.
Plasma Density: Cold and Hot Plasma
Since plasma density can vary, the properties of plasma vary depending on that density. Even a weakly ionized gas with a very low plasma density is still considered plasma and exhibits the characteristics of plasma. The plasma that is produced in the surface modification industry is usually cold plasma, meaning it is created in lower temperatures and it has a lower plasma density; usually of less than 1% or about one-hundred-million electrons per cubic centimeter. Hot plasma, which is the type associated with astrophysics, is completely ionized and has a very high plasma density of about one-trillion electrons per cubic centimeter.