Number density: The distribution with equivalent with at fixed redshift is close to a power law in column density.
Number of particles per cm3 (cf. column density).
Numbers (Dirac): ...
The Number density is the number of a particular object or species per unit volume and might be used when describing the number of electrons per cubic centimetre in a plasma or the number of stars in the core of a globular cluster.
n_s is the number density of species s (s = e,Z for electrons, ion species Z), T_e is the electron temperature in energy units, and symbols not defined here are physical constants. Note that the third bracketed factor on the right-hand side determines its units.
where is the number density and is the lifetime.
Harwit, M. New York: Springer-Verlag, p. 308, 1988.
CHARGE CARRIER NUMBER DENSITY (n) - Number density (number per volume) ofcharge carriersin a material. In conductors, electrons are free to move, so n for a conductor equals the number density of conduction electrons in the material.
where ni is the ion number density and mi is the ion mass.
 Relativistic case
The general Alfvén wave velocity is defined by  ...
~ has units cm-3 (particles per cubic centimeter).
Electromagnetic Radiation Radiation that travels through vacuous space at the speed of light and propagates by the interplay of oscillating electric and magnetic fields. This radiation has a wavelength and a frequency.
Where is the "contact surface" (the cometary ionosphere boundary) and what are the number density and chemical composition of the ions in the cometary ionosphere?
Evaluation of the photon number density, starting from the knowledge of the various cross-sections, of the ingoing flux and of the shape of the surface.2. Identification of the spatial behaviour of the cross-sections, starting from the knowledge of the exiting photon flux.3.
If the neutrinos are not massless, then they could have a larger mass density now consisting of their number density times their rest mass. Each neutrino species has a number density of n = (3/4)(4\pi)\Gamma(3)\zeta(3) (kTn/hc)3 where \Gamma is the gamma function (\Gamma(n+1) = n! ...
An image illustrating the number density of galaxies estimated to be four billion light years from the Earth. Bright areas indicate high-density regions. The brightest region in the centre corresponds to the main body of the CL0939 cluster.
(b) Within the confines of the HR diagram and/or the corresponding Color-Magnitude diagram it is also possible to give representation to the number density of stars at any particular luminosity and color/spectral-type by the use of contours or grey scale plotting.
On the basis of the numbers of cosmic rays hitting Earth, astronomers estimate the number density of cosmic rays in interstellar space to
be on the order of 10-3 particle/m3. That makes them rare, although still more numerous than interstellar dust particles.
Outside the galaxy, in intergalactic space, the number density of particles is thought to fall off to about one atom or molecule per cubic meter (10-6 / cm3).
Motions Within the Solar System ...
You can have a million-degree plasma that wouldn't have the power to boil a teaspoon of water, if the number density is low enough. Even if the CME is large, the density is really small.
See also: What is the meaning of Density, Astro, Solar, Spectrum, Sun?