Bibliographers use the term leaf for part of a folded sheet: each of its two sides makes a page. The number of leaves made from each sheet depends on the format of the book. In a folio, each sheet is folded into two leaves, or four pages; in a quarto, each sheet is folded into four leaves, or eight pages; in an octavo, each sheet is folded into eight leaves, or sixteen pages.

From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.