A measure of the distribution of power in the spectrum of white, or colorless, light, stated in terms of the kelvin temperature scale. 
The human visual system is incredibly adept at quickly correcting for changes in the color temperature of light; many kinds of light all seem “white” to us. Photographic film is not so forgiving;daylight film is made to be exposed by 5500 K light, while “indoor” film requires light with a color temperature of 3400 K (or 3200 K, for professional film). Photographs taken indoors under incandescent light on ordinary daylight film will come out orange. Photographs on indoor film taken in sunlight will be blue, as will photograph taken outdoors in shade illuminated by blue sky. To understand why a characteristic of light can be described by temperature, imagine a clear incandescent lamp connected to a dimmer. 
As the dimmer is turned up, the voltage increases and the lamp’s filament becomes warmer and warmer until it begins to glow cherry red. 
As the voltage continues to increase the filament gets hotter and hotter, glows more and more brightly, and is less and less red. 
The nature of the light is changing with the temperature of the filament. To formulate mathematical descriptions of the relationship between the temperature of the filament and the light being emitted, physicists use the concept of a blackbody, an imaginary perfect emitter and absorber of radiation. 
Most light sources emit light that is a mixture of light with different wavelengths, although some wavelengths are missing from certain kinds of light sources, such as fluorescent.
The light from a blackbody, however, is a mixture of light with a continuous range of wavelengths. 
The light is strongest at some particular wavelength. On each side of this peak, the amount of power gets progressively smaller.  The farther a wavelength is from the peak, the less power the light with that wavelength will have.
 As a blackbody gets hotter, the peak shifts toward shorter wavelengths.