Sentence:  <WSD>Folio</WSD> Page.—A page which occupies the half of a full sheet of paper, as Post-<WSD>folio</WSD>, Demy-<WSD>folio</WSD>, etc. Two pages of <WSD>folio</WSD> are imposed together as one form, four pages being a perfect sheet. Post-<WSD>folio</WSD> and Foolscap-<WSD>folio</WSD>, however, are more frequently imposed as four-page forms, and printed on Double-post and Double-foolscap paper. 
Meaning of the word marked with <WSD> tags:  A page of a book, that is, one side of a leaf of a book.