week vs hebdomad

week and hebdomad both are nouns.

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

As nouns, week and hebdomad are synonyms defined as:

week (noun)	hebdomad (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)