Definition
Delta E is a measure of color accuracy.
It represents the difference between intended color (in a file for example) and displayed color (on a screen for example)
It’s a numerical value for each color. For example, pure red is represented as slightly shifted towards blue. Delta E is the spacial (euclidean) distance between the 2 in the CIELAB color space.
It can also be used to quantify white balance accuracy. (too blue, or too orange)
It can be used to quantify the quality of applied paint in the automotive industry
Sources
https://www.rtings.com/monitor/tests/picture-quality/color-accuracy