The Seven Years War was somewhat of a global war, but it isn't considered a World War in the same way we refer to World War 1 and 2 for a few reasons, some I give below, but the main one might be that words and titles are squishy and are what we collectively agree on and it's not a "World War" because we agree it isn't and settled on the name Seven Years War*. *sort of

One thing to consider is that we don't agree on a name for the Seven Years War, in many of the American colonies it was and still is called the French and Indian War, in the northern most colonies and Canada you get the Seven Years War name, in French Canada it is the War of Conquest. It is also referred to as the Pomeranian War, the Third Silesian War and the Third Carnatic War in different places. When we look at it this way we can see how disjointed this war was, it wasn't a global unified war, but more multiple wars taking place around the same time in different locations.

Another thing to consider is that the Seven Years War involved relatively few belligerents, primarily the western European powers and their colonies. If you listed the 4 biggest world powers in 1750, it would of course be debated, but most scholars would land on, in no particular order, Britain, France, the Ottoman Empire, and the Qing Dynasty. Two of those world's biggest powers/populations were not involved. The conflict has no involvement in Africa, the Middle East, or most of Asia.

Ultimately definitions and names are what we agree on. The general definition of a World War is "a war engaged in by all or most of the principal nations of the world" (Meriam-Webster). A looser definition might is "a war involving many large nations in all different parts of the world." (Oxford) You could take it even looser and say it is a war among the great powers. The Seven Years Wars fits none of these defintions, it was waged by relatively few nations, it wasn't in all or most of world, and 2 of the great powers didn't participate.

Some people do argue that the Seven Years War is "World War Zero," personally, I would say this is a Eurocentric point of view that sees the world as Europe and her colonies, it seeks to elevate their importance and lower the importance of the rest of the globe.

We (as in the global we) have drawn a line and if we move that line to consider the Seven Years War a World War then dozens of other conflicts would also be on the list of world wars which neuters the definition and downplays the scale and level of World War 1 and 2.