Merriam-Webster Collegiate Dictionary - folio

 I. noun  (plural ~s)  Etymology: Middle English, from Latin, ablative of folium  Date: 15th century  1.  a. a leaf especially of a manuscript or book  b. a leaf number  c. a page number  d. an identifying reference in accounting used in posting  2.  a. a sheet of paper folded once  b. a case or folder for loose papers  3.  a. the size of a piece of paper cut two from a sheet; also paper or a page of this size  b. a book printed on ~ pages  c. a book of the largest size  4. 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