Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged and neutral particles of various species (electrons and ions) that responds collectively to electromagnetic forces. Such particle systems can be studied statistically, i.e., their behaviour can be described based on a limited number of global parameters instead of tracking each particle separately.

The fundamental plasma parameters in a steady state are

Using these parameters and physical constants, other plasma parameters can be derived.

All quantities are in Gaussian (cgs) units except energy ${\displaystyle E}$ and temperature ${\displaystyle T}$ which are in electronvolts. For the sake of simplicity, a single ionic species is assumed. The ion mass is expressed in units of the proton mass, ${\displaystyle \mu =m_{i}/m_{p}}$ and the ion charge in units of the elementary charge ${\displaystyle e}$, ${\displaystyle Z=q_{i}/e}$ (in the case of a fully ionized atom, ${\displaystyle Z}$ equals to the respective atomic number). The other physical quantities used are the Boltzmann constant (${\displaystyle k_{\text{B}}}$), speed of light (${\displaystyle c}$), and the Coulomb logarithm (${\displaystyle \ln \Lambda }$).

In the study of tokamaks, collisionality is a dimensionless parameter which expresses the ratio of the electron-ion collision frequency to the banana orbit frequency.

The plasma collisionality ${\displaystyle \nu ^{*}}$ is defined as $${\displaystyle \nu ^{*}=\nu _{\mathrm {ei} }\,{\sqrt {\frac {m_{\mathrm {e} }}{k_{\mathrm {B} }T_{\mathrm {e} }}}}\,\varepsilon ^{-{\frac {3}{2}}}\,qR,}$$ where ${\displaystyle \nu _{\mathrm {ei} }}$ denotes the electron-ion collision frequency, ${\displaystyle R}$ is the major radius of the plasma, ${\displaystyle \varepsilon }$ is the inverse aspect-ratio, and ${\displaystyle q}$ is the safety factor. The plasma parameters ${\displaystyle m_{\mathrm {i} }}$ and ${\displaystyle T_{\mathrm {i} }}$ denote, respectively, the mass and temperature of the ions, and ${\displaystyle k_{\mathrm {B} }}$ is the Boltzmann constant.

Temperature is a statistical quantity whose formal definition is $${\displaystyle T=\left({\frac {\partial U}{\partial S}}\right)_{V,N},}$$ or the change in internal energy with respect to entropy, holding volume and particle number constant. A practical definition comes from the fact that the atoms, molecules, or whatever particles in a system have an average kinetic energy. The average means to average over the kinetic energy of all the particles in a system.

If the velocities of a group of electrons, e.g., in a plasma, follow a Maxwell–Boltzmann distribution, then the electron temperature is defined as the temperature of that distribution. For other distributions, not assumed to be in equilibrium or have a temperature, two-thirds of the average energy is often referred to as the temperature, since for a Maxwell–Boltzmann distribution with three degrees of freedom, ${\textstyle \langle E\rangle ={\frac {3}{2}}\,k_{\text{B}}T}$.