absorption coefficient

Linear decadic ( $a$ , $K$ ) and Napierian absorption coefficients ( $α$ ) are equal to the corresponding absorbances divided by the optical path length through the sample. The molar absorption coefficients (decadic $ɛ$ , Napierian $κ$ ) are the linear absorption coefficients divided by the amount concentration.

Source:

Green Book, 2nd ed., p. 32