The purity or intensity of a colour, apart from how light or dark it is. Colour considered as itself.
Greek khroma (color, complexion, skin-tone), related to khros (skin, surface); the technical sense is a modern narrowing of the old general word for colour itself.
Splits colour from brightness so each can be asked about on its own — two colours can share the same hue and the same lightness and still differ in how loud or muted they read, and that loudness rather than the hue is what chroma names.