penumbra

noun6

1. •
   A partially shaded area around the edges of a shadow, especially an eclipse.

   • [A]ny penumbral fringe that may be detected cannot be well distinguished from the effects of the true penumbrae; […] — 1875
2. •
   A region around the edge of a sunspot, darker than the sun's surface but lighter than the middle of the sunspot.

   • The atmosphere was very clear, evidenced by the steadiness with which the mottling of the sun’s surface and the penumbrae of several spots were visible. — 1868
3. •
   (figurative) An area of uncertainty or intermediacy between two mutually exclusive states or categories.

   • These firms or businesses are not illegal in the strict sense, but there is a shadowy penumbra within which they live, and it is often convenient for the government to look the other way. — 1998
4. •
   (figurative) An area that lies on the edge of something; a fringe.

   • Thank God we are not all cowards, we have not all a low ambition, which would make men shades, poenumbrae^([sic]) of their fellows. — 1842
5. •
   A region of the brain that has lost only some of its blood supply, and retains structural integrity but has lost function.
6. •
   Something 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.