The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. 

Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture, and other fields.

In practice, color temperature is only meaningful for light sources that do, in fact, correspond somewhat closely to the radiation of some black body, i.e., those on a line from reddish-orange via yellow and more or less white to blueish white; it does not make sense to speak of the color temperature of, for example, a green or purple light. Color temperature is conventionally stated in the unit of absolute temperature, the Kelvin, which has the unit symbol K.
This relation, however, is a psychological one in contrast to the physical relation implied by Wien's displacement law, according to which the spectral peak is shifted towards shorter wavelengths (resulting in a more blueish white) for higher temperatures.