Chroma
How colourful something looks next to a grey of the same lightness.
See it in Color intelligence. Load a picture and compare the swatches: some are rich, some close to grey.
In plain words
Chroma measures how far a colour is from grey. A bright green leaf has high chroma, and a dusty olive has low chroma. It sounds like saturation, but it is measured against a grey of the same lightness. That keeps it steady as colours get lighter or darker. Designers use it to keep a set of colours equally vivid.