For me it's more the complexity of today's geopolitics that would make it impossible to call one a world war. In my example, is north korea engaged in the war? Not really, they are just lending (or selling) soldiers. Also Russia is not at war according to Putin, it's just a military operation etc...
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.
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.
The US Civil War eclipsed both in the number of casualties
I'm maybe misunderstanding you here, but as far as I know there were about 100 times as many people killed in WW2 as in the US Civil war. 60 odd million vs 600,000 or so.
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.
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.