CRI is the Color Rendering Index.
White light is formed by light that has all the colours of the rainbow. Our eyes do not see immediately if some colours are missing in the spectrum. But if a certain colour is missing in the light, we can't see that colour clearly on a certain object.
That is why colour fastness in lighting is just as important as colour temperature. If the light of a lamp is indicated as 4000K, but there is no red in the light, then red objects will hardly look red at all, but very pale. It is therefore extremely important for colour recognition that all the colours of the rainbow are equally present in the colour of the light.
The measure for colour recognition is the CRI or Color Rendering Index. A value of 0 indicates that only one very specific colour is present, whereas 100 indicates that the colour recognition corresponds to that of daylight. This CRI is defined as the average of 8 measured matt colours.
A CRI of 80 is considered to be of good quality for commercial use and at home. For use in museums or in professions where colours are very important (painters, tailors, etc.) a CRI of 90 or more is required.