a folded sheet of paper one half of which is pasted to the inside of either cover of a book, the other half to the inside edge of the first (or last) page of the book
See endpaper in American Heritage Dictionary 4
also end paper(ĕndˈpāˌpər)
noun
Either of two folded sheets of heavy paper having one half pasted to the inside front or back cover of a book and the other half pasted to the base of the first or last page. Also called end leaf.