The negative decadic logarithm of one minus absorptance as measured on a uniform sample: \[A=- \log _{10}(1- \alpha )\]
Source:
PAC, 1985, 57, 105. 'Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)' on page 116 (https://doi.org/10.1351/pac198557010105)