Gutter Margin: Margin on the “inside” of a printed book page. When books are printed, the margin on the inside is usually larger to allow for easy book handling. We assure your gutter margin when making plates for printing, free of charge.

From: ANSI/NISO/LBI STANDARD FOR LIBRARY BINDING – (inner margin, gutter margin, back margin)  The distance between the binding edge of a printed page and the printed area.