IESNA Definition: A pictorial representation of the radiant power emitted by a light source at each wavelength or band of wavelengths in the visible region of the electromagnetic spectrum (360 to 770 nanometers).

Lamp manufacturers publish SPD curves of specific light sources. The spectral make-up of a light source affects its ability to render colors "naturally", as seen in the following examples:

The incandescent light source depicted has more power in the longer wavelengths (above 650 nanometers) of the visible spectrum and therefore renders red colors most effectively.

This particular flourescent lamp has more power in the short wavelength of the visible spectrum (below 450 nanometers) than the incandescent lamp shown above, therefore blue colors appear more vivid.

Combining wavelengths in different amounts can produce light that appears white to the eye. It is possible that the light from two lamps can have different wavelength combinations and yet appear exactly the same color (same nominal correlated color temperature [CCT]) but their effects on objects may be very different (refer to CRI).