UDP, the User Datagram Protocol, is a transport layer protocol. It is unreliable in the sense that a packet is not guaranteed to arrive at its destination, but provides greater flexibility to an application that wishes to do its own flow control, and can be used in places where a connection-oriented approach is not appropriate (like multicast.) NFS, DNS, and RealPlayer use UDP.

A UDP, or Usenet Death Penalty, is a means by which site administrators and others around the world attempt to enforce the cooperative nature of usenet on an uncooperative member of that community. It is accomplished, in most cases, by what is known as an "active" UDP. An Active UDP is one in which every message posted to usenet by the offending site is canceled or failed to be propagated. This is done regardless of content, posting author, or whether or not it would normally fall under the spam cancel thresholds. This simply cuts that site, and all of its users, off from being heard by the rest of usenet. A UDP can also be focused at a narrower range of targets as well, such as "everything out of sprynet in alt.sex.*", or "Everything from user "jsmith@abusiveisp.com", or "everything out of UUNET from a particular dialup pool".
Taken from http://www.stopspam.org/usenet/faqs/udp.html

The UDP are Northern Ireland's Ulster Democratic Party, a fringe unionist party with links to the loyalist paramilitary organisation the Ulster Defence Association (UDA). They were a participant in the talks which led to the Good Friday Agreement.

UDP is used in almost all real-time games (like Quake), most streaming media (like WMA and Real Player, and anything similar that requires low-latency communications. This is because, unlike TCP, it is basically a transport layer connectionless protocol. It does not wait for re-sends, it will drop severly late packets, etc. This makes it unreliable, but fast and efficient.

UCE = U = UN*X

UDP /U-D-P/ v.,n.

[Usenet] Abbreviation for Usenet Death Penalty. Common (probably now more so than the full form), and frequently verbed. Compare IDP.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.