Definition of raison d'être noun from the Oxford Advanced American Dictionary

 

raison d'être

 noun
noun
NAmE//ˌreɪzoʊn ˈdɛtrə//
 
[singular](from French)
 
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the most important reason for someone's or something's existence Work seems to be her sole raison d'être.
See the Oxford Advanced Learner's Dictionary entry: raison d'être