All or most of the “powerful” countries being involved, usually directly. Basically take a look at the top military powers, if majority are active boots on ground, running military strategies, fighting and taking a wartime position domestically, you’ve got a world war.

Really though it’s a new-ish term, and highly subjective. WW1 was the Great War until it popped off again. For a modern thought experiment - could the war on terror be considered a world war? Much of the world’s fighting power was dragged into it to some degree, but most people would say no.

Long story short, it’s a ‘world war’ when historians decide it was.

When the Swiss pick a side.

This is one of Punxsutawney Phil’s lesser-known responsibilities.

When white Europeans are fighting each other.

When multiple great powers on each of the sides are directly* in conflict, with multiple continents being theatres of war.

* as opposed to proxy wars.

I don’t think another war will be called a world war anymore because of everyone’s ideas of a world war 3 being all out nuclear warfare.

Symptoms of a World War:

• Several major military powers and their allies are in direct conflict with one another. No proxy wars where one side fights some smaller nation that is bankrolled by the other side like Korea, Vietnam, Afghanistan, Afghanistan or Ukraine.
• It is an actual declared shooting war. No more “police actions” or “special military operations.” In the United States this requires an act of congress which hasn’t happened since 1942.
• Powers on both sides enter a total wartime economy. Food and fuel rationing, consumer goods go out of production to make more weapons, that sort of thing. In the United States, companies you don’t think of as arms manufacturers stopped what they were doing to build weapons. If Kitchenaid stops making stand mixers and starts making rifles, it’s probably World War 3.
• Actual fighting takes place in several different parts of the world for distantly related reasons. The Battle of Stalingrad, the D-Day landings at Normandy and the Battle of Guadalcanal are all considered part of the same war; present day fighting in Gaza and Ukraine are not. At least, not yet. You know how a high school history textbook has a couple chapters about the Spanish Civil War and the Japanese Invasion of Manchuria, then after the chapter end questions about China you turn the page and it says UNIT 3: WORLD WAR 2 and on the page after that it says “Chapter 18: Germany Invades Poland.” I can imagine reading a future textbook that puts Russia in Ukraine in either chapter. I think we’ll officially call it World War 3 when NATO is fighting Russia, Belarus and probably Iran for some reason in Eastern Europe and the United States, Japan, South Korea are fighting China, North Korea and probably also Russia in the Pacific, probably over Taiwan, with the Middle East as a third front akin to North Africa.
• Nukes. I think if anyone nukes anyone else we’ll call it World War 3 regardless.

Retroactively.

I’d say world wars involve multiple major powers in full total war economy. We haven’t seen any major power do that since WW2 to my knowledge. Involvement of multiple nations does not make it a world war IMO, otherwise the wars in Iraq and Afghanistan count, among many others in history. There’s also not side conflicts occurring in all the colonial possessions like in the other world wars.

When one of the sides speaks German

A World War is a war between two or more world powers.

The Pentagon has repeatedly said they have no proof North Korean soldiers are in Ukraine.

A world war would be reserved for “great powers” once again fighting each other. Since WWII, the US has been the world’s only great power, facing little opposition from the supplicated Europe and Japan. When some combination of those gang up on a perpheral nation, that is not a world war.

When a country is suddenly able to militarily pose a threat to this trinity, along with Canada and Australia and Sputh Korea, suddenly everyone in America with a cushy enough job to have anxiety as their main problem rather than exhausted living expenses constantly, global thermonuclear threat rises to the level of home burglary on their list of concerns. Yeah I think it’s pathetic, I got over fear of nuclear war when I was like eleven. Every country should have nuclear weapons. Keeps it honest.