hebdomad vs shiva

hebdomad and shiva both are nouns.

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

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

hebdomad: any period of seven consecutive days
shiva: (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, shivah.

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