Foxing is the brown spots that appear on the pages of old books, especially those printed in the 19th century. Foxing is the result of a chemical reaction, so it is more present in the parts of a book that have more chemicals used in their creation. The steel engravings used in the mid to late 19th c. frequently show foxing, even when the rest of book does not, because of the chemicals used in the printing of the engravings. Foxing also tends to be more present at the beginning and end of a volume, because those pages are closer to the chemicals used in the binding of the book.
Foxing is generally described on a scale from light or very light, meaning that there are very few spots, to heavy, meaning that they are a large number of spots.