For the purposes of visual rendering, reflectance is the wavelength a surface reflects under certain conditions—in other words, what we usually call the "color" of the surface. For example, you can think of a material's Diffuse color component as its "Diffuse reflectance."
To obtain a value for a particular pixel, a photometric renderer must consider both the reflectance of the surface material and the luminance that results from the scene illumination.