U

undo

A command that takes back the previous command, “undoing” it. For example, if you delete a word from a sentence, Undo puts it back. The shortcut for Undo is usually .

Universal Serial Bus

USB for short. USB is a technology for connecting peripherals—such as pointing devices and printers—to your computer. Apple has standardized on USB, supplanting the Apple Desktop Bus and traditional serial ports. Not only is USB 1,000 times faster than ADB and more than 50 times faster than Apple's old serial ports, it's “hot-swappable,” which means you can disconnect or connect devices without powering down or restarting the computer.

Unix

An operating system. Mac OS X is based on a version of Unix.

See also [BSD]

upload

To transfer a file to another computer over the Internet.

URL

Uniform Resource Locator. A URL is the address of a specific spot on the Internet. For example, the URL for Peachpit's home page is www.peachpit.com.

utility

A program that performs a specific task (such as managing extensions) or supplements the functions of other programs (such as an add-on spelling checker).