Definition: Offset lithography is a process used for printing on a flat surface, using printing plates. An image is transferred to a printing plate, which can be made of a variety of materials such as metal or paper. The plate is then chemically treated so that only image areas (such as type, colors, shapes and other elements) will accept ink. Water and ink is applied to the plate. Because of the chemical treatment, ink only "sticks" to the image areas, which reject the water. Areas without images reject the ink. The plate is then rolled onto a rubber cylinder applying the inked area, and in turn the rubber cylinder (or "blanket") applies the image to the paper. The system is "offset" because the plate does not come in direct contact with the paper, which preserves the quality of the plate.
David Bann. “The All New Print Production Handbook.” Watson-Guptill Publications. 2006.