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.