For plasma to exist, ionization is necessary. The term "plasma density" by itself usually refers to the electron density , that is, the number of charge-contributing electrons per unit volume. The degree of ionization  is defined as fraction of neutral particles that are ionized:
where  is the ion density and  the neutral density (in number of particles per unit volume). In the case of fully ionized matter, . Because of the quasineutrality of plasma, the electron and ion densities are related by , where  is the average ion charge (in units of the elementary charge).