What is the definition of value in ART?

I'm pretty sure value is like, the thickness or darkness of a color, or tones. Such as white being the lightest, black the darkest.

Value The lightness or darkness of tones or colors. White is the lightest value; black is the darkest. The value halfway between these extremes is called middle gray. Space An element of art by which positive and negative areas are defined or a sense of depth achieved in a work of art.