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 ~ (n) - ~ (number per volume) ofcharge carriersin a material. In conductors, electrons are free to move, so n for a conductor equals the ~ of conduction electrons in the material.
where ni is the ion ~ 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 ~ and chemical composition of the ions in the cometary ionosphere?
Evaluation of the photon ~, 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 ~ times their rest mass. Each neutrino species has a ~ 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 ~ 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 ~ 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 ~ 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 ~ 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 ~ 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?