unidirectional
a mode used in printers so they only print when the printhead is moving in a certain direction. If it prints when going both from left to right and from right to left, the system is printing bi-directional.

uniform color space
A 3-dimensional color space in which all pairs of colors that are judged to be equally different are separated by nearly equal distances

uninterruptable power supply
A device that automatically powers a computer without delay when the main power supply fails. The reverse power may be continuous, or it may be only enough protection to allow for an orderly shutdown to avoid data loss.

unit system
A counting method that assigns proportional units or width, based on the EM to each typeface character.

UNIX
Operating system that was developped by AT&T about 25 years ago. Originally it was used for very powerful computers but nowadays it is also fairly popular on personal computers. MacOS X is based on Unix and Linux resembles it a lot.

unsharp masking
A filter in scanner drivers and image manipulation programs like Photoshop that increases the contrast at the edges of density or color changes by exaggerating the differences. Inappropriate use of this filter can lead to images that look artificial.

unwanted colors
Process colors that should not be present in certain areas of a reproduction, such as cyan in yellow or magenta in greens.

Up
A general term used to indicate that multiple pages are printed in one impression on a single press sheet. “Two up” or “four up” means printing two or four pages on each sheet. The term is also used when the same image (e.g. a greeting card) is printed multiple times on a sheet.

UPC
Abbreviation for Universal Product Code, a 10-digit code used to label retail products

uplink
An earth station that can transmit a signal to a satellite above the earth.

uploading
The act of copying a file to a network or server from an individual computer

UPS
Abbreviation for Uninterruptable Power Supply, a battery that keeps a computer system running during a power outage

URL
Abbreviation for Universal Resource Locator, an address that identifies a page on the web (http:/www.prepressure.com/ is the URL of this web site)

USB
abbreviation for Universal Serial Bus, a standard for data transmission between a computer and peripherals like digital cameras, keyboards, scanners or mice.

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