Electromagnetic radiation at wavelengths of about 380 nm to 780 nm, which are visible to the human eye.
The based measurements with a few subjects definds spectral sensitivity is based on the photometric sizes with the conversion of Watt (physical value) to lumens (brightness).
Of course there are individueal variations in the spectral sensitivity and can be serious due to aging, cataract surgery of characteristics such as red-green color blindness.