∙ A partially shaded area around the edges of a shadow, especially an eclipse.
∙ A region around the edge of a sunspot, darker than the sun's surface but lighter than the middle of the sunspot. astronomy
∙ An area of uncertainty or intermediacy between two mutually exclusive states or categories. figuratively
∙ An area that lies on the edge of something; a fringe. figuratively
∙ 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.
∙ (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. medicine