Definitions
1.nounA partially shaded area around the edges of a shadow, especially an eclipse.
2.nounA region around the edge of a sunspot, darker than the sun's surface but lighter than the middle of the sunspot.
3.nounAn area of uncertainty or intermediacy between two mutually exclusive states or categories.
4.nounAn area that lies on the edge of something; a fringe.
5.nounSomething related to, connected to, and implied by, the existence of something else that is necessary for the second thing to be full and complete in its essential aspects.
6.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.