 Number density

The number density is the number of particles per unit volume. For a homogeneous species with $g$ internal states and phase-space distribution $f(\mathbf{p})$ in natural units,

Number density is a physical quantity that describes the concentration of a number of particles, such as atoms, molecules, or other entities, in a given volume of space. It is typically represented by the symbol \( n \) and is measured in units of particles per unit volume, such as particles per cubic meter (m⁻³) or particles per cubic centimeter (cm⁻³).