idée fixe

The man you have just been talking to, poor fellow, is one of the strongest cases of pure idee fixe that we have.

Happy lily, never to be saddled with an idee fixe, never to be in the grip of a monomania for happiness or love or fulfilment.

Germans are the 'idee fixe', and without an 'idee fixe', as you know, there's no such thing as religion.

He never doubted Claridge Pasha's return; it was une idee fixe.

Darrell thus put into Lady Adela's head, he remained there, and became an idee fixe.