UDF – Universal Disk Format, for DVD volumes.

UDP (User Datagram Protocol)-A communications method that uses the Internet Protocol IP to send a data unit called a datagram from one computer to another in a network. Network applications that have very small data units to exchange may use UDP rather than TCP.

UFS – UNIX File System format, a data storage format. For more information, see Mac OS X 10.0-Choosing UFS or Mac OS Extended HFS Plus Formatting Knowledge Base article 25316

UID (User ID) – A number that uniquely identifies a user. Mac OS X computers use the UID to keep track of a user’s directory and file ownership.

Unicast – The one-to-one form of streaming. If RTSP is provided, the user can move freely from point to point in an on-demand movie.

Unicode – A 16-bit character encoding standard developed by the Unicode Consortium between 1988 and 1991. By representing two bytes to represent each character, Unicode enables almost of the written languages of the world to be represented using a single character set.

Uniquing – A mechanism to ensure that, within a given context, only one object is associated with each row in the database.

Up Arrow – A key you can press in some applications to make the cursor move up one line.

URL – Uniform Resource Locator. A term for the address of an Internet site or other resource.

USB – Universal Serial Bus. An industry-wide peripheral bus standard that supports a data speed of 12 megabits per second, and that accommodates a wide variety of devices. Most new computers and peripheral devices are equipped with USB.

User Group – A computer club whose members share programs they’ve written and information they’ve learned.

User ID – A number that identifies you as a subscriber to an information service.

User Interface – The way a computer application communicates with you.

User Name – The long name for a user, sometimes referred to as the user’s real name.

Utilities or Utility Program – A set of applications that controls and manipulates the information on disks.