rai‧son d'être /ˌreɪzɒn ˈdetrə $ -zoʊn-/
noun [singular][
Date: 1800-1900;
Language: French;
Origin: 'reason of being']
the reason why something exists, why someone does something etc:
Commerce was the town’s raison d'être. [TahlilGaran] Dictionary of Contemporary English ▲