The term "folio", from the Latin folium (leaf), apparently has three interconnected but distinct meanings in the world of books and printing. It is firstly a term for a common method of arranging sheets of paper into book form, folding the sheet only once, and a term for a book made in this way. Secondly it is a general term for a sheet, leaf or page in (especially) manuscripts and old books, and thirdly an approximate term for the size of a book, and for a book of this size.
Firstly a folio (abbreviated fo or 2°) is a book or pamphlet made up of one or more full sheets of paper, on each of which four pages of text are printed, two on each side; each sheet is then folded once to produce two leaves. Each leaf of a folio book thus is one half the size of the original sheet. Ordinarily, additional printed folio sheets would be inserted inside one another to form a group or "gathering" of leaves prior to binding the book.
Secondly "folio" is used in terms of page numbering for some books and most manuscripts that are bound but without page numbers as an equivalent of "page" (both sides), "sheet" or "leaf", using recto and verso to designate the first and second sides, and (unlike the usage in printing) disregarding whether the leaf concerned is actually physically still joined with another leaf. This usually appears abbreviated: "f26r." means the first side of the 26th leaf in a book. This will be on the right hand side of the opening of any book composed in a script that is read from left-to-right, such as Latin (as used in English), Cyrillic, or Greek, and will be opposite for books composed in a script that is read from right-to-left, such as Hebrew and Arabic.
Thirdly folio is also used as an approximate term for a size of book, typically about 15 inches (38 cm) tall, and as such does not necessarily indicate the actual printing format of the books, which may even be unknown as is the case for many modern books. Other common book formats are quarto and octavo, which are both also printing formats, involving two and three folds in the sheet respectively. Famous folios (in both senses) include the Gutenberg Bible, printed in about 1455, and the First Folio collected edition of Shakespeare's plays, printed in 1623; however their actual size is rather different.