Referring to the edges of a bound book that have been folded over the page edges. This means by design, not by abuse. The binding materal is usually some flexable material, such as limp leather or paper.
yellowed A defect of the paper in a book, where the paper has discolored. It is a visible sign of decay. Good quality paper resists yellowing. Unfortunately, the bibliophile cannot usually tell what paper is prone to yellowing and which is not, at least not until the yellowing occurs.
Virtually all modern paper is manufactured from wood pulp. One of the components of wood is lignin, and unless the lignin is thoroughly washed from the paper, it will, over time, acidify and chemically attack the paper fiber.
The amount and rate of discoloration is going to vary with storage conditions. Higher temperature and higher humidity will hasten yellowing. Since environmental conditions are a major factor, it's not surprising that yellowing usually commences at the edges of the printed page, and slowly intrudes to the center. This is why you often see the catalog notation: "edges yellowed". Toning is the same as yellowing, but implies a very mild case. At the opposite extreme you have browning.
The product of certain publishers, especially publishers of juveniles, seem to be especially prone to paper discoloration. Saalfield is a particular example, and, unfortunately for those of us who remember the classic science fiction of the fifties with great fondness, so is Gnome Press.

This definition is in context to Book Collecting. See more contextual defintions for Yapped.