[physics]
The proportion of incident radiant energy that is reflected by a surface. Reflectance varies according to the wavelengths of the incident radiant energy and the color and composition of the surface.
[photogrammetry]
In imagery, the ratio of light reflected from an object or that is captured by a target above the object. This ratio is wavelength-dependent, and the value's range is between 0 and 1 or can be expressed as a percentage.