A color temperature (TC) of an electromagnetic radiation source is a measure of the temperature based upon a color index, which is based on the ratio of the brightness of the source of two different bands. In effect, it is the temperature of the black-body spectrum for which that color index best matches that of the source.
Note that the term color temperature is used differently outside astronomy. There is some relation, but the criteria used undoubtedly differs. For example, descriptions of light produced by artificial lighting (e.g., light bulbs) may use to the (approximate) temperature of a black body that is perceived as the same color as the light source.