Number density
（重定向自Photon density）
In physics, astronomy, chemistry, biology and geography, number density (symbol: n or ρN) is an intensive quantity used to describe the degree of concentration of countable objects (particles, molecules, phonons, cells, galaxies, etc.) in physical space: three-dimensional volume number density, two-dimensional area number density, or one-dimensional line number density. Population density is an example of areal number density. The term number concentration (symbol: C, to avoid confusion with amount of substance n) is sometimes used in chemistry for the same quantity, particularly when comparing with other concentrations.