penumbra

Noun

penumbra (plural penumbras or penumbrae or (obsolete) penumbræ)

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 penumbræ; […]1875 October, E. Neison [pseudonym; Edmund Neville Nevill], “On the Condition of the Atmospheres of the Planets”, in William Crookes, editor, The Quarterly Journal of Science, […], volume V (New...
   • The other places see the penumbra of the moon's shadow fall on the earth, so the eclipse is partial, and part of the sun's disc is still visible.2011, Galen C. Duree, Jr., Optics for Dummies, Wiley Publishing, Inc., published 2011, →ISBN, page 61
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 penumbræ of several spots were visible.1868 December 8, Henry Pratt, “The Transit of Mercury”, in The Astronomical Register: […], volume VII, number 73 (January 1869), London: J. D. Potter, […], →OCLC, “Correspondence” section, page 20
   • The velocities found in the penumbræ of spots generally increase with the distance from the centre of radiation, the motion being zero at some point in the umbra, and accelerating outwards.1921, Kodaikanal Observatory Bulletin, volume III, New Delhi: Manager of Publications, →OCLC, page 173
3. (figuratively) 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, Debraj Ray, Development Economics, Princeton University Press, →ISBN, pages 346–347
   • […] God chose to descend into the realm of human imperfection, where the light of truth is spare and must exist in the penumbra of partial knowledge mixed with partial ignorance.2010, Denis Farkasfalvy, Inspiration and Interpretation: A Theological Introduction to Sacred Scripture, The Catholic University of America Press, published 2010, →ISBN, page 188

   Synonyms:

   grey area

4. (figuratively) 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, pœnumbræ^([sic]) of their fellows.1842, [George B. Loring], England Opposed to Slavery, or Some Remarks upon “An Examination into the Real Causes of the War Against the United States, and an Appeal to the Other Powers of Europe...
   • Whilst the orthodox, de-charismatized churches steadily lose influence and support and the new cults develop, in the religious penumbra there have persisted, during the last century, echoes of charisma.1975, Bryan R. Wilson, The Noble Savages: The Primitive Origins of Charisma and Its Contemporary Survival, Quantum Books, published 1975, →ISBN, page 116

   Synonyms:

   periphery, outskirt

5. 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.
   • The foregoing [United States Supreme Court] cases suggest that specific guarantees in the Bill of Rights have penumbras, formed by emanations from those guarantees that help give them life and substance.1965, William O. Douglas, Griswold v. Connecticut, United States Reports, 381 U.S. 479
6. (preceded by "ischaemic") A region of the brain that has lost only some of its blood supply, and retains structural integrity but has lost function.