hebdomad vs week

hebdomad and week both are nouns.

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

As nouns, hebdomad and week are synonyms defined as:

hebdomad (noun)	week (noun)
any period of seven consecutive days	any period of seven consecutive days
	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)