I think of a quilt as having the back and top stitched together, whether the stitching creates squares or just one central panel; when you pick a quilt up, the filling doesn't all fall to one end; and it is complete in itself, it does not have a separate cover.
A duvet is the other thing.
I am from the South East.
Bizarrely, although 'duvet' is a French word; the primary meaning is 'down, soft feathers' ; the French don't call one a duvet at all. To them it's 'une couette' (as I discovered when trying buy one !)