Very generally, "leaves" refers to the pages of a book, as in the common phrase, "loose-leaf pages."
A leaf is a single sheet bound in a book, and a leaf has two pages. The first page that you read on a leaf is the recto page, and you turn it over to read the verso page.
Books are classified into sizes based on the height versus width of their leaves. See "folio," "quarto," "duodecimo," and "octavo" to learn more about book sizes.
A folded grouping of leaves is a "gathering."
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