Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.
Color temperature has been described most simply as a method of describing the color characteristics of light, usually either warm (yellowish) or cool (bluish) and measuring it in degrees Kelvin (°K).
That's a little too simple to be of more than introductory value.
A more technical definition assigns a numerical value to the color emitted by a light source, measured in degrees Kelvin. The Kelvin Color Temperature scale imagines a black body object--- (such as a lamp filament) being heated. At some point, the object will get hot enough to begin to glow. As it gets hotter its glowing color will shift, move from deep reds, such as a low burning fire would give, to oranges & yellows, all the way up to white hot. Light sources that glow this way are called "incandescent radiators", and the advantage to them is that they have a continuous spectrum. This means that they radiate light energy at all wavelengths of their spectrum, therefore rendering all the colors of a scene being lit by them, equally. Only light from sources functioning this way can meet the truest definition of color temperature.