A shadow is a dark area on a bright surface. It is caused by something blocking a source of light. A shadow's outline, called a silhouette, will have the same shape as the object blocking the light.

Depending on the light source, or sources, a wide range of effects can be produced. A point source of light casts only a simple shadow, called an "umbra". If there is more than one light source, there will be several shadows, with the overlapping parts darker, and various combinations of brightnesses or even colors. The more diffuse the lighting is, the softer and more indistinct the shadow outlines become, until they disappear. The lighting of an overcast sky produces few visible shadows.

Astronomical shadows are usually sharp. This is because in space the light usually does not go through an atmosphere.