The number density is the number of particles per unit volume. For a homogeneous species with internal states and phase-space distribution in natural units,
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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).