The magnitude of transmission is given as transmittance. This is the quotient of the transmitted energy, i.e. the energy transmitted by the medium, and the energy in front of the medium.
The transmittance or transmission coefficient is denoted by T or the Greek tau and is a dimensionless number between 1 and 0. If all the energy is transmitted by the medium the transmittance is 1, if no energy is transmitted the value is 0. There is a direct mathematical relationship between the transmittance, reflection coefficient and absorption coefficient: the sum of the three is 1.
In optical systems, the transmittance stands for the light transmission of the material.