Definition of lightest:
(n) :
(physics, uncountable) Electromagnetic radiation in the wavelength range visible to the human eye (about 400–750 nanometers): visible light.
(n) :
(by extension) Electromagnetic radiation in the wavelength range visible to the human eye or in nearby ranges (infrared or ultraviolet radiation).
(n) :
(by extension, less commonly) Electromagnetic radiation of any wavelength.
(n) :
(countable) A source of illumination.
(n) :
A lightbulb or similar light-emitting device, regardless of whether it is lit.
(n) :
A traffic light, or (by extension) an intersection controlled by traffic lights.