Edition is a bibliographical term, meaning “all the copies of a book printed from one setting of type or from printing plates made from the typesetting” (Matthew J. Bruccoli). It shouldn’t be confused with issue, printing, or state.
In common parlance, edition often refers to the work of a single editor or team of editors, in whatever bibliographical version it appears, as long as the text is the same. So, for instance, the Twickenham Edition of Pope applies both to the twelve-volume monster and the handy-dandy one-volume condensed version.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.