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.

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.

When the Swiss pick a side.

When white Europeans are fighting each other.

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

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.

Major powers in direct conflict with each other.

For instance, the Spanish Civil War is seen as a precursor to World War II, but it isn’t considered a part of World War II because different sides supported different belligerents, including direct military action, the conflict remained in Spain.

This is part of the reason why NATO nations have not provided direct military action to support Ukraine, as it would lead to a likely escalation to World War III.

Two or more Great Powers on each side

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

2001 probably when usa decided to declare war on the whole world

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

Callously, when the survivors look back and decide to call it one. As far as I know there isn’t an agreed upon definition.

WW1 was originally called the War to End All Wars, I think, by many at the time. WW2 eclipsed it by taking place on at least 3 continents and across every ocean. Both are also known by other names that depend on the region. The US Civil War eclipsed both in the number of casualties. The Ukraine war isn’t likely to break records like that.