A
really big book, often with a
thick,
heavy-duty binding. Often used in
fantasy stories to refer to
magic spellbooks, and
metaphorically for any book larger than others of its kind. (See
The Wheel of Time.)
The largest tome I own is Merriam-Webster's New International Dictionary, Second Edition which is over 5 inches thick worth of 9-by-12 pages. This book also comes in an India paper edition which is thinner.