1) Centerfold 2) Centrefold 3) Foldout 4) Insert 1) Foldout 2) Page 3) Swindler, so to speak 
oversized page in a book
Where a page, usually illustrated, folds out to accommodate the image that is larger than the books page size.
An oversize page where both sides fold into the gutter in overlapping layers.
Double or triple-size pages, generally in magazines, that fold out into a large advertisement.
A gatefold or foldout is an oversize page in a book or magazine that is folded in.