folio

The definition of a folio is a large sheet of paper folded in the middle, or a large book containing sheets of paper.

(noun)

A folder which can protect papers inside is an example of a folio.
A piece of paper folded over and forming four pages in a book is an example of a folio.

See folio in Webster's New World College Dictionary

noun pl. folios

Bookkeeping a page of a ledger, or facing pages with the same number
Law a set number of words (100 in the U.S., 72 or 90 in England) considered as a unit of measuring the length of a legal or official document
Printing
1. a large sheet of paper folded once, so that it forms two leaves, or four pages, of a book, manuscript, etc.
2. a large size of book, about 12 by 15 inches, made of sheets folded in this way
3. a leaf of a manuscript, book, etc. numbered on only one side
4. the number of a page in a book, etc.

Origin: ME < L (in) folio, (in) a sheet, abl. of folium, a leaf (in LL, leaf of paper): see foliate

adjective

having sheets folded once; of the size of a folio

transitive verb folioed, folioing

to number the pages of (a book, etc.) consecutively; page

See folio in American Heritage Dictionary 4

noun pl. fo·li·os

a. A large sheet of paper folded once in the middle, making two leaves or four pages of a book or manuscript.
b. A book or manuscript of the largest common size, usually about 38 centimeters (15 inches) in height, consisting of such folded sheets.
a. A leaf of a book numbered only on the front side.
b. A number on such a leaf.
c. A page number.
Accounting A page in a ledger or two facing pages that are assigned a single number.
Law A specific number of words used as a unit for measuring the length of the text of a document.