idée fixe /ˌiːdeɪ ˈfiːks 
/ ▸ noun (plural idées fixes /ˌiːdeɪ ˈfiːks /) an idea or desire that dominates the mind; an obsession: I believe we need to challenge the idée fixe that only the appearance of youth is valuable.
– ORIGIN French, literally ‘fixed idea’.
EnglishOxford Dictionary of English
