week vs rag week

week and rag week both are nouns.

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

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

week: any period of seven consecutive days
rag week: a week at British universities during which side-shows and processions of floats are organized to raise money for charities

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

week (noun)	rag week (noun)
any period of seven consecutive days	a week at British universities during which side-shows and processions of floats are organized to raise money for charities
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)