Value
How light or dark a colour is, with the colour itself set aside.
See it in FX engine. Set Color mode to Grayscale.
In plain words
Every colour has a value: how light or dark it looks. Yellow has a high value and navy blue a low one. Turn a picture grey and value is all that is left. Artists squint at a scene to see values without the distraction of colour. A picture with a clear range of values reads well even from far away.