A sheet, as bibliographers use the term, is a large piece of paper, often about the size of an unfolded newspaper sheet. A sheet is folded before it is bound: if it's folded once to make two leaves or four pages, it makes a folio; if it's folded twice to make four leaves or eight pages, it makes a quarto; if it's folded three times to make eight leaves or sixteen pages, it makes an octavo.

From the Guide to Literary Terms by Jack Lynch.
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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.