A more thorough explanation:

Definition: A folio is a leaf of paper or parchment that is numbered only on the front. It includes both sides of the leaf, or two pages, with the letters "a" and "b" (or "r" and "v") added to show which of the two pages was intended.

For example, in a book, page 1 might be labeled "1a" on the front and "1b" on the back. This is because the page is actually one folio, or one leaf of paper folded in half to create two pages.

In legal documents, a folio is also used as a method of measurement. In England, 72 or 90 words formed a folio, while in the United States, 100 words were used. This helped to standardize the length of legal documents and make them easier to read and understand.

Finally, a folio can also refer to a large book that is created by folding a sheet of paper only once in the binding to form two leaves, making available four pages (both sides of each leaf). This type of book is often used for art books or other large-format publications.

Overall, the term "folio" refers to a specific type of page or leaf in a book or legal document, as well as a type of book format. Understanding the different meanings of this term can help readers better understand the structure and organization of printed materials.