Color Temperature
Describing the color of white light by the temperature of a glowing ideal object that would emit it, from candle amber near 1,900 K to overcast blue above 6,500 K.
Color temperature describes the hue of a white light source by naming the temperature, in kelvin, of an idealized blackbody that would glow the same color. Heat any object enough and it emits light whose spectrum depends only on its temperature: dull red near 1,000 K, orange-yellow around 2,000 K, neutral white near 5,500 K, and bluish above that. The path those colors trace across a chromaticity diagram is the Planckian locus.
The naming runs backwards from intuition. "Warm" light, the amber of a candle (about 1,900 K) or an incandescent bulb (about 2,700 K), comes from a cooler radiator. "Cool" bluish daylight (5,000–6,500 K) is named for a hotter one. Incandescent lamps actually are blackbodies, more or less. Fluorescent and LED sources are not. They make light by entirely different means, and their rating is a correlated color temperature: the blackbody temperature they look closest to. A second number, Duv, says how far off the locus they sit, a little green or a little magenta. Two lamps with the same CCT can look visibly different for that reason, and for differences in color rendering.
Dimming used to change color for free. An incandescent filament dimmed to 10% runs cooler and glows redder, so a dimmed room turned amber, like firelight. LEDs hold the same color at every level, so a dimmed cool-white LED reads as gray and clinical. Dim-to-warm lamps reintroduce the old behavior on purpose by blending warmer emitters as brightness falls. Arie Kruithof's 1941 curve, which proposed that low light levels feel pleasant only at warm temperatures and bright ones only at cool temperatures, is shaky as science but describes the effect most people notice.
Color temperature also matters for the body. Cooler, bluer light at night delays the Circadian Rhythm more than warm light of the same brightness, the basis of Circadian Lighting and of night modes on screens. Displays and cameras use the same scale for their white point: the reference white a Color Gamut is built around, typically 6,500 K (D65) for screens.