A vague, poorly defined area or idea
This usage comes from the euphemistic use of the word, which originally referred to actual, shadowed areas. Made infamous by judicial activists on the Supreme Court.
Greetings traveller. Who am I? Perhap’ you have met me twixt sleep and wank, in the penumbra of uncertainty you call “unconsciousness”.
— Garth Marenghi, Garth Marenghi’s Dark Place
William James used to preach the “will to believe.” For my part, I should wish to preach the “will to doubt.” None of our beliefs are quite true; all at least have a penumbra of vagueness and error.
— Bertrand Russel, Skeptical Essay
Stalking and martyrdom are acts that are seen, to their executors, to fall under the penumbra of “love”.
— Kaz Vorpal
Paena is Latin for “almost”, umbra for “shadow”…an umbrella is a “little shade”