Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged particles that responds collectively to electromagnetic forces. Plasma typically takes the form of neutral gas-like clouds or charged ion beams, but may also include dust and grains. The behaviour of such particle systems can be studied statistically. Fundamental plasma parameters All quantities are in Gaussian (cgs) units except energy and temperature which are in electronvolts. The ion mass is expressed in units of the proton mass and the ion charge in units of the elementary charge (in the case of a fully ionized atom, equals to the respective atomic number). The other physical quantities used are the Boltzmann constant (), speed of light (), and the Coulomb logarithm (). Frequencies Lengths Velocities Dimensionless number of particles in a Debye sphere Alfvén speed to speed of light ratio electron plasma frequency to gyrofrequency ratio ion plasma frequency to gyrofrequency ratio thermal pressure to magnetic pressure ratio, or beta, β magnetic field energy to ion rest energy ratio Collisionality 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 is defined as where denotes the electron-ion collision frequency, is the major radius of the plasma, is the inverse aspect-ratio, and is the safety factor. The plasma parameters and denote, respectively, the mass and temperature of the ions, and is the Boltzmann constant. Electron temperature Temperature is a statistical quantity whose formal definition is 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, . The SI unit of temperature is the kelvin (K), but using the above relation the electron temperature is often expressed in terms of the energy unit electronvolt (eV). Each kelvin