Short for Color Rendering Index. On a scale from 1 to 100, CRI measures how an artificial light source compares to natural white light, with 100 being essentially identical to natural light.
In the days of incandescent light bulbs, CRI was not such an issue. With LEDs and advanced electronics, however, the light spectrum can vary quite a lot. The light from an LED can be very different from natural light.
This can have a dramatic impact on the colors you see. CRI is like the treble and bass on your stereo. You can turn a yellow banana blue with an LED. If color is really important, such as on a painting job, the luminaires should have a high CRI. Atexor SLAM® luminaires, for example, use LEDs with at least 80 CRI.