penumbra vs shadow

penumbra and shadow both are nouns.

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

As nouns, shadow is a hypernym of penumbra; that is, shadow is a word with a broader meaning than penumbra:

penumbra (noun)	shadow (noun)
a fringe region of partial shadow around an umbra	refuge from danger or observation
	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

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