gatefold
noun [ C ]
MARKETING uk/ˈɡeɪtfəʊld/ usa cover for a record or set of two or more music CDs that is folded in the middle and opens out so that it is twice as big. Gatefolds are also sometimes used in the covers of books or magazines:
I personally prefer the gatefold arrangement, whereby the back cover folds out to reveal yet more colour photographs.