A peaked cap as used by many military organisations and the odd civilian services such as the police, post office.
This one is clearly RAF at least that what the hat badge would indicate, a quick search says its airman's no.1 SD hat badge whatever that means.
The visor or peak is made in this case of plastic, but originally the peaks would be polished leather.
Peaked cap also known as Forage Caps, Barrack Covers or Combination Cap.
