Merriam-Webster's Collegiate® Dictionary, 11th Edition
FOLIO
I. noun (plural folios)
Etymology: Middle English, from Latin, ablative of folium
Date: 15th century
1. a leaf especially of a manuscript or book, a leaf number, a page number, an identifying reference in accounting used in posting,
2. a sheet of paper folded once, a case or folder for loose papers,
3. the size of a piece of paper cut two from a sheet, a book printed on folio pages, a book of the largest size, a certain number of words taken as a unit or division in a document for purposes of measurement or reference,
II. transitive verb
Date: 1858
to put a serial number on each leaf or page of