- Astronomy.
- the partial or imperfect shadow outside the complete shadow of an opaque body, as a planet, where the light from the source of illumination is only partly cut off.
- the grayish marginal portion of a sunspot.
- a shadowy, indefinite, or marginal area: a penumbra of secrecy.
- a right or set of rights implicit but not expressed in the U.S. Constitution or Bill of Rights: The principle of the penumbra protects against an imperfect, or incomplete, enumeration of rights.
- a fringe region of half shadow resulting from the partial obstruction of light by an opaque object
- the lighter and outer region of a sunspot
- the point or area in which light and shade blend
