Light
The degree of illumination defining an object's visibility, and its contrast against surrounding darkness.
In practice
Light decides what can be seen and how forms read. Designers work with it when photographing a product, rendering a 3D object or placing lamps in an exhibition. A single side light carves a chair's curves into strong shadows, while flat, even light makes the same chair look like a paper cutout. On a dark app screen, a bright card reads as nearer and more important because it stands out against the darkness.
Often confused with Tone
Light is the illumination falling on or coming from a scene; tone is the resulting lightness of a particular area as it appears in the image.
Try this
Photograph a mug lit by one lamp from the side, then from directly above, and compare how solid it looks.