The nip between the blanket cylinder and the impression cylinder on an offset press or, in other words, the space in which a press sheet is passed to be printed. (See Offset Lithography: Printing Unit.)
On a gravure press, the impression nip is the line of contact between the image-carrying gravure cylinder and the resilient impression roller, through which the substrate passes. (See Gravure and Impression Roller.)
On a flexographic press, the impression nip is the line between the plate cylinder and the impression cylinder, through which the substrate passes. See Flexography.