Sentence:  <WSD>Folio</WSD>.—The running number of the pages of a work. When there is no running title, the <WSD>folio</WSD> is placed in the centre of the head-line; when there is a running title, at the outside corner—the even <WSD>folio</WSD> on the left, the odd on the right. The preface, contents, index, and all introductory matter, usually have separate <WSD>folios</WSD> inserted in Roman lower-case numerals. 
Meaning of the word marked with <WSD> tags:  A page number. The even folios are on the left-hand pages and the odd folios on the right-hand pages.