Is there a term for those half-cocked hats with large flower-like embellishments British high society seems to favor? I'm seeing quite a lot of them watching a live stream of the Royal Wedding, and would like to call them by their proper name.
You mean the Cocktail Hats?
The list is taken from here.
I think you could call them Derby Hats (although this is also a synonym for bowler hat when applied to men).
Depending on which side of the pond you stand, that could mean either