Color Temperature

Colour temperature is a standard method of describing colours for use in a range of situations and with different equipment. Colour temperatures are normally expressed in units called kelvins (K). Note that the term degrees kelvin is often used but is not technically correct (see below).

Technically speaking...
Colour temperature means the temperature of an ideal black body radiator at which the colour of the light source and the black body are identical. (A black body is a theoretical radiator and absorber of energy at all electromagnetic wavelengths.)

Degrees kelvin
According to the The International System of Units (SI) , colour temperatures are stated in kelvins, not in degrees Kelvin. The "degrees" part of the name was made obsolete in 1967. However, the "degrees" reference has remained in common use.