U

Unicasting: Communication between a sender and a single receiver over a network. For example, an email message sent from one person to another.

Uplink: The communication link from a transmitting earth station to a satellite.

Upload: To send a file from one computer or server to another.

URI (uniform resource identifier): Name and address of information--text, graphics, audio, video, and so forth--on the Internet. A URI usually identifies the application used to access the resource, the machine the resource is located on, and the file name of the resource. A Webpage address or URL is the most commonly used type of URI.

URL (uniform resource locator): The address of a page on the World Wide Web. For example, .

Usability: The measure of how effectively, efficiently, and easily a person can navigate an interface, find information on it, and achieve his or her goals.