As bibliographers use the term, a page is one side of a leaf, which is in turn part of a sheet. The number of pages produced from each sheet depends on the format: 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.
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.