shadow vs penumbra

shadow and penumbra both are nouns.

Word	Noun	Adjective	Verb	Adverb
shadow	Yes	No	Yes	No
penumbra	Yes	No	No	No

As nouns, penumbra is a hyponym of shadow; that is, penumbra is a word with a more specific, narrower meaning than shadow:

Other hyponyms of shadow include umbra.

shadow (noun)	penumbra (noun)
refuge from danger or observation	a fringe region of partial shadow around an umbra
something existing in perception only
an indication that something has been present
a premonition of something adverse
an unilluminated area
a spy employed to follow someone and report their movements
an inseparable companion
a dominating and pervasive presence
shade within clear boundaries

shadow (verb)	penumbra (verb)
follow, usually without the person's knowledge
make appear small by comparison
cast a shadow over