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.
Symptoms of a World War: