noun, plural rai·sons d'ê·tre [rey-zohnz de-truh; French re-zawn de-truh] /ˈreɪ zoʊnz ˈdɛ trə; French rɛ zɔ̃ ˈdɛ trə/.
Origin of raison d'être
noun plural raisons d'être (rɛzɔ̃ dɛtrə)
"excuse for being," 1864, first recorded in letter of J.S. Mill, from French raison d'être, literally "rational grounds for existence."
A basic, essential purpose; a reason to exist: “Professor Naylor argues that in the nuclear age, infantry forces have lost their raison d'être.” From French, meaning “reason for being.”