—n. 1. a sheet of paper folded once to make two leaves, or four pages, of a book or manuscript.
2. a volume having pages of the largest size, formerly made from such a sheet.
3. a leaf of a manuscript or book numbered only on the front side.
4. Print. a. (in a book) the number of each page.
b. (in a newspaper) the number of each page together with the date and the name of the newspaper.
5. Bookkeeping.a page of an account book or a left-hand page and a right-hand page facing each other and having the same serial number.
6. Law.a certain number of words, in the U.S. generally 100, taken as a unit for computing the length of a document.
—adj. pertaining to or having the format of a folio: a folio volume.
—v.t. 1. to number each leaf or page of.
2. Law.to mark each folio in (a pleading or the like) with the proper number.