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Number density (often symbolized as n) is a term for the number of objects per unit volume.
N
n = —
V
for number density n, number of objects (e.g., a particle number) N, and volume V. The term is often used for atoms or molecules, e.g., in a gas, in contrast to other measures such as mass density. The term, number density, can also be used for larger objects, such as for the stars within a unit volume.
Number densities are sometimes cited for just one of the types of objects within the volume, e.g., that of just the oxygen molecules within Earth atmosphere at sea level. An alternative to such a number density is a partial pressure, a calculation of the pressure contribution of one type of object, e.g., calculated by the ideal gas law. An example partial pressure is electron pressure, which the Saha equation can be adapted to use.