Quarto a. : Having four leaves to the sheet; of the form or size of a quarto.

Quarto n. : Originally, a book of the size of the fourth of sheet of printing paper; a size leaves; in present usage, a book of a square or nearly square form, and usually of large size.

Example:
What text of Hamlet is usually performed? The Second Quarto, the First Folio, or a conflation of both?

Taxonomy
four leaves, quarto edition, Gutenberg Bible, octavo, free encyclopedia, pamphlet, sheets, Shakespeare quartos, Search, sheet of paper, second quarto, Q1, gatherings, two times, leaf, fragment, bibliographers, demy.

Defination from the web:
Quarto (abbreviated 4to or 4°) is a technical term describing the format of a book, which refers to the size of leaves produced from folding a ... The earliest known European printed book is a quarto, the Sibyllenbuch, believed to have been printed by Gutenberg in 1452-53, before the ...