Correlated colour temperature

Correlated colour temperature or CCT describes the colour impression or colour temperature of a light source compared to the ideal radiation of a black-body. Nowadays, the most common and frequently used light sources don't work with thermal radiation, as the incandescent bulb does. This means that the emitted light does not follow the form of the black-body curve. The CCT of a light source (e.g. LED) describes the colour temperature of the source which most closely matches the colour temperature of a black-body radiator as perceived by the human eye. 

Tip from a lighting designer 

Knowing the CCT and the movement of the lighting temperatures along the black body is important when planning dim to warm or tunable white light. CCT dimming is difficult since there are many parameters that influence the result, like binning and ageing of the LEDs. Often, the most frequently used temperatures are below or above the black-body curve, resulting in unnatural-looking light.