radiant power specularly reflected from the surface of a system, P refl, divided by the incident power, P 0: \[\rho =\frac{P_{\mathrm{refl}}}{P_{0}}\]
Source:
PAC, 1985, 57, 105. 'Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)' on page 115 (https://doi.org/10.1351/pac198557010105)