An Emission Coefficient (often indicated as j) is a measure of a substance's Electromagnetic Radiation (EMR) Emission in a particular direction, increasing the total EMR leaving in that direction over that passing through. Such coefficients are used for modeling the effect of gas on passing EMR. It is a ratio of the EMR emitted to that entering the substance over a standard length through the substance. (This can be treated as a ratio because this emission is dependent on the incoming radiation: it stems from the Atomic Excitation caused by Absorption of the incoming radiation.) It will depend upon the substance and its density. It is the inverse of an Absorption Coefficient.
These coefficients are basic to models of Stellar Structure and of gas clouds.