1) Coefficient 2) Physical quantity 3) Transmission 
• (n.) Transmission.
the ratio of transmitted power to incident power for given conditions of spectral composition, polarization and geometrical distribution NOTE - In optics, frequently expressed as transmittance density or as a percentage. In communication applications, generally expressed in decibels.ratio of the transmitted radiant or luminous flux to the incident flux in the given conditions unit: 1 NOTE - See Note 1 to 845-04-63.
the ratio of radiant energy transmitted through a substance to the total radiant energy incident on its surface. In solar technology, transmittance is always affected by the thickness and composition of the glass cover plates on the collector or window, and to a major extent by the angle of incidence between the sun's rays and a line normal to the ...
<radiobiology> Ratio of the radiant power transmitted by an object to the incident radiant power. ... See: reflectivity. ... (09 Oct 1997) ... 
The fraction of the light that passes through an object
The ratio of the radiant power transmitted through a material to the incident radiant power. Transmittance is usually expressed as a percent. A filter with a 50% transmittance (at a specific wavelength) will absorb half of the light incident on it and allow half of it to pass through it.
The fraction of the light that passes through an object. 
[n] - the fraction of radiant energy that passes through a substance
transmittance The fraction of percentage of a particular frequency or wavelength of electronmagnetic radiation that passes through a substance without being absorbed or reflected.
the amount of light that passes through a medium
transmission noun the fraction of radiant energy that passes through a substance
(of Light) That fraction of the emitted light of a given wavelength which is not reflected or absorbed, but which passes through a material or object.