from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The page size obtained by folding a whole sheet into four leaves.
- n. A book composed of pages of this size.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A size of paper (7.5"-10" x 10"-12.5")(190-254 x 254-312 mm). Formed by folding and cutting one of several standard sizes of paper (15"-20" x 20"-25")(381-508 x 508-635 mm) twice to form 4 leaves (eight sides).
- n. A book size, corresponding to the paper size.
from the GNU version of the Collaborative International Dictionary of English
- adj. Having four leaves to the sheet; of the form or size of a 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.
from The Century Dictionary and Cyclopedia
- n. A size of book in which the leaf is one fourth of a described or implied size of paper.
- Noting the size of a book in which a sheet makes four leaves: as, a quarto volume; being of the size or shape of the leaves of a quarto; as, quarto paper; a quarto edition.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. the size of a book whose pages are made by folding a sheet of paper twice to form four leaves
They form six volumes in quarto; and as Basil is the worst, so
Ephorus, in quarto historiarum libro, orbem terrarum inter Scythas,
Latin, Lipsiae, 1766, in quarto, with the learned notes of Kochler and Reiske, and some extracts of geography and natural history from
Shakespeare's "Pericles" exists only in a lousy quarto, which is so badly transcribed scholars assume it was done by someone jotting down the script from memory after having seen the show (the early modern equivalent of the grainy pirated videos you can buy on the subway).
The remainder of the first period has filled two volumes in quarto, being the third, fourth, fifth, and sixth volumes of the octavo edition.]
Although the book is of the size called quarto, the method of printing must have been page by page, so it is doubtful that each sheet was folded twice in the usual quarto manner, but more probable that it was printed four pages to a sheet of paper approximately 9 1/8 by 14 inches, which was folded once.
I print the book at my own expense, in quarto, which is to be sold for six shillings, with the music.
In May, 1905, was celebrated at St. Louis, Mo., his twenty-fifth anniversary as bishop in the church, which proceedings of this Silver Jubilee have been published in a journal called the quarto-centennial that will do ample justice to this great hero which, with our feeble pen we are unable to do.
India, being used in what we should call quarto sheets, and in Farther
 Mr. Croker quotes a note by Malone to show that in the catalogue of Steevens's Library this book is described as a quarto, corio turcico foliis deauratis.