A score (0–100) for how accurately a light source shows colours compared with natural daylight.
CRI (Ra) is a CIE-defined metric comparing a light source's rendering of a standard set of test colours against a reference illuminant of the same colour temperature.
CRI and colour temperature measure different things — a light can be a flattering warm colour temperature but still have poor CRI, rendering colours inaccurately.
Many cheap LED bulbs have lower CRI than daylight or high-quality bulbs, which is part of why paint samples can look 'off' under them.
A low-CRI bulb is a common, under-recognised reason a paint colour looks different at home than in a shop or outdoors.