Offset printing is an indirect flat printing process widely used in book, newspaper and packaging printing. The physical basis is the different surface structure of the printing plate. The printing parts of the plate are “lipophilic” they attract oils and repel water, therefore absorbing ink. The non-printing part of the plate is “hydrophilic”, i.e. it attracts water.
The parts thus previously wetted with water therefore do not absorb ink.
Indirect printing and the English term “offset” derived from it means that printing is not done directly from the printing plate onto the paper, but the ink is first transferred via another roller, the blanket cylinder.