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Printing Glossary

This glossary of printing terms was created by people working in today's printing industry and is brought to you by It has been revised and edited and we have rewritten some technical descriptions in every day language to help the non technical person. Any suggestions that you may have on how we can improve this glossary will be carefully considered. Please send your comments and any new definitions to us at

Offset Printing

Offset Printing is any form of printing that uses an intermediate carrier to transfer the image from the original image-carrier (such as a plate) to the substrate. Although "offset printing" can refer to any printing process, it is most commonly used as an alternate term for offset lithography.

Onion Skin

Onionskin or onion skin is a thin, light-weight, strong, often translucent paper. It was usually used with carbon paper for typing duplicates in a typewriter, for permanent records where low bulk was important, or for airmail correspondence. It typically has a 9 pound basis weight, and may be white or canary colored.
In the typewriter era, onion skin often had a deeply-textured cockle finish which allowed for easier erasure of typing mistakes, but other glazed and unglazed finishes were also available then and may be more common today.


In papermaking, opacity is a property of paper that describes the amount of light which is transmitted through it. Paper that has a high degree of opacity does not let much light pass through it, while paper that has a low degree of opacity is more translucent, or allows much light to pass through it. A paper's opacity determines the extent to which printing on a particular side of paper will be visible from the reverse side (called show-through).
In optics, the extent to which an object or surface will impede the transmission of light through it. A completely opaque object is one which allows no light to pass through it.

Open Prepress Interface

A workflow protocol developed by Aldus Corporation used in electronic prepress to link desktop publishing systems and high-end CEPS. Essentially, high-resolution color images are stored on a central network server, to which all the workstations are connected. Low-resolution files are sent by the server to individual computers working on page layout. The low-res images are imported into the page (in a kind of FPO way), positioned, and comments sent back to the OPI server provide specific cropping, scaling, positioning, and color information about the image. The server's PostScript driver inserts the proper instructions into the PostScript code. When the page is ultimately output to an imagesetter connected to the network, the high-resolution image is swapped for the low-res one, and the indicated instructions as to cropping, etc., are executed.
OPI is useful for minimizing high-resolution-file travel on networks; their large file size can make traffic screech to a halt. And by utilizing only low-resolution viewfiles on workstations, processing speed is increased. The efficacy of OPI is contingent upon the use on the workstations of OPI-compatible software; many page layout programs are increasingly including support for OPI, although some OPI specifications for color separation haven't been effectively nailed down yet. Although OPI is often compared to DCS, the latter is strictly a color separation protocol, while the former is more of a workflow protocol.

Outline Halftone

A means of halftone reproduction in which a specific portion of the image is outlined (or silhouetted) by removing the halftone dots which surround it.

Over Run

An Over Run is additional material printed beyond the request of the order. Each printing company's overage policy varies depending on the size and scope of the project.


To place one or more images on top of another image, each image being essentially a separate layer. Images can be overlayed either on a mechanical board by means of sheets of tissue paper or acetate, or on a computer monitor.

Overlay Proof

One of two primary types of color proof, comprising a set of thin, transparent sheets of plastic or film, each of which contains one of the four process colors, cyan, magenta, yellow, and black, overlayed on top of each other to simulate the appearance of the final, full-color reproduction. Two common trade names which are used as synonyms for overlay color proofs are Color Key and NAPS.