What is an "edition"?: Actions by an "editor" and the rest of the printshop crew resulting in a series of artifacts
Think about the root verb of the noun, "edition," either a set of objects or the name of a process which we more commonly call "editing." In the digital era, editing typically may be done on screen, though experienced editors often prefer to edit printed paper copies if the edition will be printed. In the hand-press era, editors typically turned authors' manuscript text into "fair copies" prepared for the type setter unless they were editing a previously printed text, perhaps correcting errors and adding missing material. Manuscripts may contain editorial alterations of the exemplars from which they were copied, but that is not the same thing as the practice of editing a manuscript, or a previous printed book, to create a new "edition." Bibliography calls "an edition" all the copies produced by one setting of the type. If the type setting is preserved after one turn on the press and remounted on the press to print more copies, perhaps if the books sells better than expected, later copies are called the "second impression" or "third impression," etc., of the first edition.
Every manuscript copy of a text will be unique because of copying mistakes and intentional changes, differences in scribal hands, illuminations, etc. All copies of every print edition bear the unique version of the text produced by the editorial decisions of the printer/publisher and the typesetter. You will also see claims that every copy of a printed edition also is unique because of minute differences in its "state" (i.e., the precisely measured and counted way the leaves came off the press in the moments and days when the book was made), but those differences are far more subtle than differences between editions, which (as Williams and Abbott will explain) can make huge differences in what we see when we try to interpret a text. That deserves calm consideration when we're sitting with pages from an edition in our hands.