What, exactly, makes a war a "world war"?

 

It can't be because the war had battles across every continent of the world, because the First World War didn't have any battles on the Americas, and the Second World War didn't have any battles on Antarctica. And yet those two wars are considered world wars.

 

Is it because those two world wars had battles across more than one continent? There have been numerous wars (besides the first and second world wars) that had battles across multiple continents. The Crimean War, the Napoleonic Wars, French Revolutionary Wars, American Revolutionary War, Seven Years' War, War of the Austrian Succession, Anglo-Spanish wars, War of the Spanish Succession, Nine Years' War, Dutch-Portuguese War, Ottoman-Hapsburg wars, Byzantine-Ottoman Wars, Crusades, Arab-Byzantine Wars, Byzantine-Sassanid wars, Great Roman Civil War, First Mithridatic War, Roman-Persian Wars, Roman-Seleucid War, First Punic War, Second Punic War, Wars of the Diadochi, Wars of Alexander the Great... just to name a few. And yet none of these are considered "world wars".

 

Is it because the two world wars had enormous casualties? The First World War is believed to have had somewhere between 16,563,868 – 40,000,000 casualties. There have been numerous wars with as many (if not more) casualties than the First World War. The Taiping Rebellion (20,000,000 – 100,000,000 casualties), Qing dynasty conquest of the Ming dynasty (25,000,000 – 25,000,000), Conquests of Timur (15,000,000 – 20,000,000), Mongol conquests (30,000,000 – 40,000,000), An Lushan Rebellion (13,000,000 – 36,000,000), and the Three Kingdoms wars of China (36,000,000 – 40,000,000).

 

There are some who are now saying that the Cold War and War on Terror should be considered world wars.

 

So what is it? What makes a war a "world war"?