The most important reason, the justification, or purpose for someone or something’s existence. What you live to do–or why you do (important) things. More colloquially, the “be-all and end-all”.
From French: “reason for being”, which in turn comes from the Latin ratiō (reason) and esse (to be). First appears in English in a letter written in 1864 by John Stuart Mill: “Modes of speech which have a real raison d’être.”
Still pronounced as in French: [rey-zohn de-truh]