Penumbra
The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a point source only the umbra is cast.
These names are most often used for the shadows cast by celestial bodies, though they are sometimes used to describe levels of darkness, such as in sunspots.
Umbra
The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a point source only the umbra is cast.
These names are most often used for the shadows cast by celestial bodies, though they are sometimes used to describe levels of darkness, such as in sunspots.
Penumbra (noun)
A partially shaded area around the edges of a shadow, especially an eclipse.
Penumbra (noun)
A region around the edge of a sunspot, darker than the sun’s surface but lighter than the middle of the sunspot.
Penumbra (noun)
An area of uncertainty or intermediacy between two mutually exclusive states or categories.
Penumbra (noun)
An area that lies on the edge of something; a fringe.
Penumbra (noun)
Something implied by, the existence of something else that is necessary for the second thing to be full and complete in its essential aspects.
Penumbra (noun)
(in “ischaemic penumbra”, after a stroke) A region of the brain that has lost only some of its blood supply, and retains structural integrity but has lost function.
Umbra (noun)
The fully shaded inner region of a shadow cast by an opaque object.
Umbra (noun)
The central region of a sunspot.
Umbra (noun)
A shadow.
Umbra (noun)
An uninvited guest brought along by one who was invited.