The penumbra (Latin for mid-shadow) is the portion of a shadow that results from the source of illumination being only partially blocked. Penumbras only occur when the source of light is not a point-source. As the sun is a visible disc, solar shadows have penumbras. The penumbra part of a shadow is contrasted with the umbra part of the shadow in which the light source is completely blocked.

Also, the penumbra is the lighter, fuzzier outer edge of a shadow. The shadow is darkest at the umbra, near the object, and weakest at the penumbra, at the edge.

The penumbra is also the ring-shaped semi-bright region of a sunspot that surrounds the darker umbra.

The Penumbra is also a region in the Umbra, the spirit world of the World of Darkness line of role-playing games.

An area in which something exists to a lesser or uncertain degree: “The First Amendment has a penumbra where privacy is protected from governmental intrusion” (Joseph A. Califano, Jr.).

See also: eclipse.