A way of writing colours as lightness, chroma and hue that tracks how different the colours look more evenly than HSL.
The cylindrical form of OKLab, a perceptual colour space published by Björn Ottosson in 2020 and adopted in CSS Color Level 4 as oklch(). Equal steps in L look like roughly equal steps in lightness across hues, unlike HSL.
OKLCH can describe colours outside any screen's gamut; a browser has to map them in, so an oklch() value is not a promise of what will display.
oklch(70% 0.15 145) is a mid green whose lightness matches oklch(70% 0.15 30) far more closely than the HSL equivalents would.
It makes building even colour scales, themes and accessible palettes far more predictable.