A printing technique based on lithography, a form of planography.
Basic lithography depends on inks that are receptive to grease and repellant to water. When the printing plate is made, the image is made grease-receptive and water-repellant, and the non-printing areas are made water-receptive and ink-repellant. (Traditionally, this is done by drawing the print image on a limestone block using a grease pencil.) The plate is mounted on the plate cylinder of the press which, as it rotates, comes into contact first with rollers wet by a water or dampening solution and then with rollers wet with ink. The dampening solution wets the non-printing areas of the plate and prevents the ink from wetting these areas. The ink wets the image areas.
What differentiates offset lithography is that the plate cylinder rolls against a rubber-covered blanket cylinder. The image is transferred to blanket cylinder and then to the paper as it passes between it and the impression cylinder.
Transferring the image from the plate to a rubber blanket before transfer to the paper is called the offset principle. Letterpress and gravure can also be printed this way, although the term offset has become synonymous with lithography.
A major advantage of the offset principle is that the rubber blanket creates a clearer impression on a wide variety of paper surfaces and other materials