Real Dictionary

folio
countable noun: A folio is a book made with paper of a large size, used especially in the early centuries of European printing.

folio in American English
a page of a ledger, or facing pages with the same number
having sheets folded once; of the size of a folio
to number the pages of (a book, etc.) consecutively; page
adjective: pertaining to or having the format of a folio
noun: a sheet of paper folded once to make two leaves, or four pages, of a book or manuscript
transitive verb: to number each leaf or page of

folio in British English
adjective: relating to or having the format of a folio
noun: a sheet of paper folded in half to make two leaves for a book or manuscript
verb: to number the leaves of (a book) consecutively