Offline
Pertaining to equipment not under direct control of the central processing unit.

Off-press proofs
Proofs made by photomechanical or digital means in less time and at lower cost than press proofs.

Off-set
Off-set, or set-off, is the undesirable transfer of ink from one freshly printed sheet to the back of the next and is usually caused by too much ink being applied to the substrate and/or ink that is slow to dry.

Offset
An indirect printing process, usually referring to offset lithography. The term offset comes from the fact that ink is transferred to paper from a blanket that carries an impression from the printing plate.

Offset lithography (photolithography, offset)
The most common form of lithographic printing in which the image area and the nonimage area exist on the same plane (plate), separated by chemical repulsion. To print, the ink is "offset" (transferred) from the plate onto a rubber blanket and then to the paper.

Offset paper
Also known as book paper. General description of any paper primarily suited for offset printing. Can be coated or uncoated. Characterized by strength, dimensional stability, lack of curl and freedom from foreign surface material. Finish is typically vellum or smooth.

Offset press (sheet fed)
Indirect rotary press with plate cylinder, blanket cylinder and an impression cylinder.

On demand printing
Typically refers to printing that is done as needed, not in large quantities and then stored. Files are stored digitally on a computer system so that they are current and can be printing quickly and cost effectively.

One-up, two-up, etc.
Printing one (two, three, etc.) impressions of a job at a time.

Onionskin
A lightweight, cockle finish paper used for making copies of correspondence.

Opacity
Paper property that measures degree to which paper stops light form passing through. Also opaqueness. The higher the opacity, the less likely that the printing on one side will be visible from the other side. Opacity is not always determined by thickness or weight of a sheet, but by the amount and type of fibers and fillers used to minimize the amount of "show-through" in a sheet from one side to the other or to the next sheet. For envelopes, inside tints can be used to compensate for low-opacity papers.

Opaque ink
An ink that conceals all color beneath it.

Open-end (envelopes)
The open-end envelope has its opening on the shorter dimension and usually has a center seam on the back.

Open-side (envelopes)
The open-side envelope has its opening on the longer dimension and has either diagonal or double side seams.

Optical brightness
Optical brighteners or fluorescent dyes are extensively used to make high, bright blue-white papers. They absorb invisible ultraviolet light and convert to visible light, falling into the blue to violet portion of the spectrum, which is then reflected back to the eye.

Optical whitener
A dye that is added to the fiber stock or applied to the paper surface at the size press to enhance its brightness.

Orange peel
A granular surface on coated or printed paper that resembles orange peel.

Out-of-register
(1) Two or more colors are not in the proper position when printed; register does not "match." (2) Descriptive of pages on both sides of the sheet which do not back up accurately.

Out-of-square
Refers to paper that has been trimmed improperly thus causing the corners to be less or more than 90 degrees. This leads to difficulty during the printing process and often results in misregister of the printed piece. Also called off-square.

Outline halftone (silhouette halftone)
A halftone image that is outlined by removing the dots that surround it.

Outside side seam (envelopes)
Single seam construction on open-end envelopes, where the seam is glued on the top or back panel. Can be right or left side (ROSS, LOSS).

Overinked
Describes printing when too much ink has been used, resulting in heavy print that tends to blur toward the back of the press sheet.

Overlay
In artwork, a transparent covering over the copy where color break, instructions or corrections are marked. Also, instead of dots coexisting on the same sheet of acetate, each color - magenta (red), cyan (blue), yellow and black - is represented on a different acetate overlay. Since this acetate is virtually transparent, the combination of four overlays will make a full-color image.

Overpressure
Too much pressure, causing ink to tend to plug letters, especially halftone dots.

Overprinting
Double printing. Also, printing over an area that already has been printed.

Overrun
Quantity of paper that is manufactured beyond the quantity specified. In printing, copies printed in excess of the specified quantity.

Oxidation
A chemical reaction that hardens the ink vehicle and makes the film of ink reasonably rub-proof. The process of combining with oxygen.