fixed idea

noun

1. a persistent or obsessing idea, often delusional, that can, in extreme form, be a symptom of psychosis.

noun

1. an idea, esp one of an obsessional nature, that is persistently maintained and not subject to changeAlso: idée fixe

n.

1. An often unreasonable idea or feeling that persists despite evidence to the contrary or efforts to ignore it; an obsession.idée fixe