A measure of how warm (yellow/orange) or cool (blue-white) a light source looks, usually given in Kelvin.
Correlated colour temperature (CCT) describes a light source's appearance relative to an idealised blackbody radiator; lower Kelvin values (around 2700K) look warm/yellow, higher values (5000K+) look cool/blue-white.
Counterintuitively, lower Kelvin numbers are 'warmer'-looking light and higher numbers are 'cooler'-looking, the opposite of everyday temperature intuition.
A 2700K bulb gives a warm, cosy light; a 5000K bulb gives a cooler, more clinical light.
The same wall colour can look noticeably different under a 2700K bulb versus a 5000K bulb — see the lighting guide.