nan idea, esp one of an obsessional nature, that is persistently maintained and not subject to change. Also: idée fixe
i•dée fixe
(i deɪ ˈfiks) n.
pl. i•dées fixes (i deɪ ˈfiks) a persistent or obsessing idea, often delusional, that in extreme form can be a symptom of psychosis. Also called fixed idea.EncyclopediaSeeIDEAThesaurusSeeidea