What makes a World War?

I have a very general question. What makes a World War? I thought of swore as a kid I was told that x amount of countries involved in a war made a world war or x amount of countries fighting eachother made a world war. The USA fighting Iraq does seem like a war but does not really involve the world. I would think the majority of countries would need to be involved to be considered a world war. Over 50 percent or at least over 50 percent of large enough countries holding arms and a population of "X" amount.

Anyone know or would agree with my later comments?

You have a food claim, but I do not think that the USA is at with Iraq, but there is a war on terror all over, that takes up for everything.

A world war is when there are two hemispheres at war, western and eastern, if the US was to get involved in the Israeli hezbollah conflict it woudl still be a war on terro, but if Iran and the USA went at it it would be a world war technically speaking.

but lets hope it does not come to that