Over recent years there has been considerable confusion over the use of the term "e-book", and this article examines the variety of definitions used to date while proposing a definitive construct. Beginning by examining the definitions of "book", the paper moves on to consider the essential element of a book--the content, and to examine publishing and structural aspects of e-books, as well as their place in libraries, before arriving at a final definition. The definition and its derivation embrace all of the issues that affect the way in which e-books are understood and used today. In conclusion, the article looks at both the genesis of e-books, and the stage of acceptance and adoption that they have reached, with brief reference to 3rd-generation e-book readers available at the time of writing. (Contains 1 table and 3 figures.)