A User Identification Number used to identify a specific user to the ICQ IM system formerly operated by Mirabilis, now owned by AOL.  UIN's are normally associated with a variety of voluntarily-submitted information such as name, nickname, birthdate, web page, and assorted contact information, amongst other things. UIN's are usually from five to nine digits in length--which is one reason ICQ is sometimes less-preferred than AOL Instant Messanger, which uses more easily remembered words and phrases for the same purpose, despite the fact the ICQ interface is generally much "slicker" and less intrusive.

What's truly amazing is that a UIN of five or six digits can be bought outright on eBay. In one auction, someone paid $305.00 for a five-digit UIN!

This sort of thing isn't limited to ICQ; some personal ads in Manhattan say "(212) only please", even though the area codes are overlaid and therefore 212 says nothing about location.

But it's pretty damn sad anyhow. Repeat after me: It's just a number.

REFERENCE: http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=1616884277 ($305 for 5-digit UIN)