an idea, esp one of an obsessional nature, that is persistently maintained and not subject to change
Also: idée fixe
fixed idea in American English
noun
a persistent or obsessing idea, often delusional, that can, in extreme form, be a symptom of psychosis
Word origin
[1820–30]This word is first recorded in the period 1820–30. Other words that entered Englishat around the same time include: acetate, blouse, boomerang, karma, myth
Examples of 'fixed idea' in a sentence
fixed idea
All men have a fixed idea of how they'd behave in one; ask around.
Times, Sunday Times (2010)
In other words, is it actually him you want, or is it a fixed idea of how you think the future should look?
Times, Sunday Times (2009)
Is it that you've got a fixed idea of the sort of work you can do?
The Sun (2010)
He says that he has no fixed ideas about his career anyway.
Times, Sunday Times (2007)
As the year turns, some fixed ideas start to change and you see how much is there for you.