Unicode
A 16 bit ISO 10646 character set. It can accommodate way more characters that ASCII, thus allowing for easier internationalization.

Unique Visitor
A real visitor to a web site. Web servers record the IP addresses of each visitor, and this is used to determine the number of real people who have visited a web site. If for example, someone visits twenty pages within a web site, the server will count only one unique visitor (because the page accesses are all associated with the same IP address) but twenty page accesses.

UNIX
A text-based, multitasking Operating System suitable for Web and network administration. Unix has spawned numerous popular spinoffs, including Linux and FreeBSD. Most Unix-based Operating Systems are open source, meaning the source code to the Operating System has been made freely available to the public for modification. Most Web hosting companies and resellers offer Unix-based hosting in some capacity or other.

Unqualified Domain Name
A domain name that is intentionally written incompletely with the understanding that some ancestor domain name will be appended to form a fully qualified domain name.

UPS - Uninterruptible Power Supply
UPS keeps the server running on a battery for several minutes after a power outage, allowing for a clean shutdown without loss of data. UPS can also shield the server from line voltage spikes and drops.

Uptime
Refers to the amount of time within a specific period that a hosting provider's system is active or available for servicing site visitors. All hosts must have some downtime for routine maintenance or unexpected outages. The most reliable hosts will guarantee 99.9% uptime.

URL - Uniform Resource Locator
URL is a way of addressing used for world wide web. An URL consist of the type of service (protocol), then the host name and then the file on the host.

Usenet
Network of all the newsgroups in the Internet.

UTP - Unshielded Twisted Pair
Similar to the STP, but without the shielding metal layer. It is more prone to interference but is less bulky that the STP cables.

UUencode
It is a method of sending binary files using email - similar in purpose to MIME.