What is an "impression"?

        When an editor determines the edition of a manuscript of typescript text, the typesetter loads the chase with lines of type in their proper order for printing, and the chase is locked up to prevent the type from moving around.  This "setting" of the type is the "first edition."  The first pages printed from that setting, from the beginning to the end of the press run (continuous operation of the press without dismounting the type) is the first impression of the first edition.

        After the end of the press run, as few as 300 to 500 copies in the fifteenth-century or 1000 to 3000 copies in the Early Modern and Machine Press eras, the setting of the type would be taken off the press to make way for some other project.  Then a decision must be made whether to leave the type locked up for another impression if the first should sell out, or to separate, sort, and restore all the individual type pieces to the type cases to make them available for another book.  If the type is left in its first setting and put back on the press to produce more books at a later date, that is the "second impression" of the "first edition."

        In 1739, William Ged, a Scottish goldsmith invented a system of copying en masse a setting of single or multiple pages of type on a single impressed plate, so that the individual types could be redistributed to print new books while still preserving a form of the edition's setting in case more copies were needed.  He did not succeed in making the process profitable, however the French printers began using the process at the turn of the century.  In England, it was not until 1804 that Lord Stanhope, who also invented the first mass produced "machine press," reinvented the process now known as "stereotype" printing, called in French, "clich."  Between 1810 and 1840, stereotype printing became one of the most common ways to print large quantities of popular books.  Increasingly, only fine quality, small-press-run books were printed with individually set type.  Each stereotype page looked exactly like every other, from which the metaphor of "stereotyped thinking" arose, but for printing, this significantly increased the number of  books which could be "printed on demand" without having to reset their type from scratch.  (Also see the U. Iowa "Lucile Project" web page reproducing a C19 article describing the process.)