In some ways, every published book is "an edition."
This term "edition" is sometimes used to describe a published book when the work was prepared for publication by someone other than the author. Usually, editions are edited versions of an original work previously published.
If you are working with a book that is not the first edition - it is important to include information on the edition you use for your research in your citations, so that page numbers for specific references are accurate. You can elect to include the original date of publication in your record, but this is not always required.
For more information on entering information on different editions, see subsquent examples in this section of the styleguide; also see the subsection on Editions in the "Entering Bibliographic Information" section.
Additional examples of editions: