Which is one of the main ingredients of a roux?

HAL 9000, wrong. Roux is specifically flour and fat (e.g. butter) only, adding milk makes it a white sauce

baymax, Roux is fat and flour. To make white sauce, you add milk to the roux.

well technically, milk is the biggest amount of a white sauce. but I suppose you could allow flour!

I only make bechamel sauce or eclairs from my roux so debated between flour and milk...luckily pressed flour.