dict=CALD&key=39521&ph=on noun [C usually singular] (US ALSO printing) all the copies of a book that have been printed at the same time without any changes being made: This is the second impression of the encyclopedia. Basically, it's another word for a print run. For example, a publisher prints 10,000 copies of the first edition (the first impression), and when he's sold them all he'll print another 10,000 copies of the first edition (the second impression).
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impression (BOOKS) http://dictionary.cambridge.org/define.asp?dict=CALD&key=39521&ph=on
noun [C usually singular] (US ALSO printing)
all the copies of a book that have been printed