Value

I often ask students if they know what value means in terms of art. I always emphasize the 'in art' part, but I always get the same answer: How much something is worth. No, that's what students know from prior knowledge, in fact, it's the only definition of value they know. I define value (when referring to art) as the lightness and darkness of a color. I don't think it's very concise though...I'm searching for a better definition.

Here are some others I found: Shadows, darkness, contrasts and light are all values in artwork.
An element of art that refers to luminance or luminosity — the lightness or darkness of a color. This is important in any polychromatic image, but it can be more apparent when an image is monochromatic, as in many drawings, woodcuts, lithographs, and photographs. This is commonly the case in much sculpture and architecture too. 
The relative lightness or darkness of a hue. Black is low value. White is a high value.
the lightness or darkness of a color; contrasts between light and dark.
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. Because a painted image is physically two-dimensional, a painter must have some tool to create a false, but convincing illusion of three-dimensionality. Value is that tool. The effects of value are most easily seen in a black and white drawing. In such a drawing, one can find a range of tones from pure black, across a spectrum of gray, ending in pure white. By using such a scale of tones, a painter is able to recreate in two dimensions the effects of light and shadow on a three-dimensional object. In a painting, such tones are usually found in spectrums of color instead of gray, but the effect is the same. Value is extremely important to a painter because without its proper use it would be impossible for a painter to create convincingly realistic imagery. It’s also a useful tool for adding further definition to forms, of which line alone is incapable of doing. Value also works in conjunction with contrast.

Okay, so I guess my definition isn't so bad. I need to gather some good lessons to teach value because it's so important. I think it's the most important concept to grasp, because it makes a huuuge difference in drawing, painting and photography.

I'm going to gather them together and post here when I'm done.