shivah vs hebdomad

shivah and hebdomad both are nouns.

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

As nouns, hebdomad is a hypernym of shivah; that is, hebdomad is a word with a broader meaning than shivah:

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

Other hypernyms of shivah include week.

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