HSL
A colour model describing colour by hue, saturation and lightness, widely used for targeted colour editing.
In depth
HSL matters in practice because it matches how photographers think: you rarely want to change red, green and blue values, but you often want a specific colour to be a different shade, more intense or brighter. Editing tools expose it as per-colour-band sliders, letting a single hue range be adjusted while the rest of the frame is untouched.
Examples in practice
Dropping the luminance of the blue band alone deepens a sky without darkening anything else in the landscape.