Value
How light or dark a colour is, on a scale from black to white, independent of hue.
Technical definition
In the HSV colour model, value is the maximum of the red, green and blue channels — distinct from HSL lightness, which averages the maximum and minimum channel.
Common confusion
HSV value and HSL lightness are both called 'brightness' informally but are calculated differently and can disagree slightly for the same colour.
Example
A pure, fully saturated blue and a pale sky blue can have similar HSV value but very different HSL lightness.
Why it matters
Value contrast (not hue) is usually what makes text readable against a background — see the contrast checker.
Related terms
Sources
- CSS Color Module Level 4 — W3C (World Wide Web Consortium)