Ø

A vowel appearing in the Norwegian alphabet (among others) but not in English. ø is pronounced "uh," like the "u" in "hurt."

The symbol ø appears in the other users nodelet to label users who have turned off the chatterbox.

Character info:
ISO Latin 1 character code: 248 (uppercase is 216)
Html entity: ø (uppercase is Ø)
TeX character: \o (uppercase \O)

Ø is only used in Danish and Norwegian to my best knowledge. The Swedes liked the German letters more, and therefore they have ä and ö instead of æ and ø (æ not to be mistaken for the French œ which is actually pronounced just like the Danish/Norwegian ø (and Swedish/German ö).
As you sometimes find yourself in a situation where you don't trust the special characters on your keyboard, you can use oe instead, which most people in all five countries understand (Oe in uppercase).
Another nice useless fact is that the word ø actually means island in Danish. And the danes actually have a meaning for the word å as well - which they share with the Norwegians AFAIR)

The letter ø (Ø in capital) is used in Norwegian and Danish. It is a vowel and is pronounced like the U in the English word "burden" or like the I in "Sir" (or even like the O in "word"!). It is the 28th letter of the Norwegian alphabet.

Example of its use in Norwegian:
Prøver du å ødelegge døra? (Are you trying to destroy that door?).

In mathematics, this symbol is used to represent the empty set, the set which has no elements. For those of you who have not studied set theory, a set looks like this: { 1 , 2 , 57.3 , 0 } it can have numbers and stuff in it. You could also have a set of variables, functions, and actually, a set of anything. (ie. a set of noders: { --OutpostMir-- , dem bones , Jet-Poop } ). Well, anyway, back to the empty set: The empty set is seen quite often when taking the subsets of a set. The subsets of the set { 1 , 2 , 3 } are as follows: { 1 , 2 , 3 } , { 1 , 2 } , { 2 , 3 } , { 1 , 3 } , { 1 } , { 2 } , { 3 } , and ø. Understand? No? Good.