week vs shiva

week and shiva both are nouns.

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

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

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

week (noun)	shiva (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)