week vs shivah

week and shivah both are nouns.

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

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

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

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

week (noun)	shivah (noun)
any period of seven consecutive days	(Judaism) a period of seven days of mourning after the death of close relative
hours or days of work in a calendar week
a period of seven consecutive days (usually reckoned to be from Sunday to Saturday, or from Monday to Sunday)