hebdomad vs shivah

hebdomad and shivah both are nouns.

Word	Noun	Adjective	Verb	Adverb
hebdomad	Yes	No	No	No
shivah	Yes	No	No	No

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

hebdomad: any period of seven consecutive days
shivah: (Judaism) a period of seven days of mourning after the death of close relative

Other hyponyms of hebdomad include week from Monday, rag, rag week, Holy Week, Passion Week, shibah, shiva.

hebdomad (noun)	shivah (noun)
any period of seven consecutive days	(Judaism) a period of seven days of mourning after the death of close relative