raison d'être
Concise Oxford English Dictionary © 2008 Oxford University Press:
raison d'être /ˌreɪzɒ̃ ˈdɛtrə/
▶noun (pl. raisons d'être pronunc. same) the most important reason or purpose for someone or something's existence.
– origin Fr., lit. ‘reason for being’.
'raison d'être' also found in these Oxford entries:

