A world war is a war affecting the majority of the world's most powerful and popular nations. World wars span multiple continents and can last for years before showing any signs of slowing. In fact, as a world war progresses, nations devote more and more of their resources to the conflict, resulting in ever-escalating destruction. World wars result in unparalleled devastation to almost every country involved, with loss of life which can only be estimated. Total economic collapse, widespread famine, and rampant disease continue to add to the death toll even many years after the fighting has ended, even for the winning side.

The term has usually been applied to two conflicts of unprecedented scale that occurred during the 20th century: World War I (1914–1918), and World War II (1939–1945).