1. (in spectroscopy) The molar absorption coefficient (symbol ε ) is a quantity that characterizes the absorption of light (or any other type of electromagnetic radiation) as it passes through a sample of the absorbing material. It has the dimensions of 1/(concentration x length). e is dependent on the frequency of the incident light; its highest value occurs where the absorption is most intense. Since absorption bands usually spread over a range of values of the frequency v it is useful to define a quantity called the integrated absorption coefficient, A, which is the integral of all the absorption coefficients in the band, i.e. A= ʃε(v)dv. This quantity characterizes the intensity of a transition. It was formerly called the extinction coefficient.
2. The volume of a given gas, measured at standard temperature and pressure, that will dissolve in unit volume of a given liquid.