A world war is an international conflict that involves most or all of the worlds major powers. There is no formal definition of what a "World War" is, though many have been proposed. However, there are some criteria that are commonly used to classify a war as a world war:

• The conflict takes place between multiple nations across the globe.
• The war involves many large nations in all different parts of the world.
• The conflict is fought by most of the planets nations, across continents and for years.
• The war is engaged in by all or most of the principal nations of the world.

The term "world war" is conventionally reserved for two major international conflicts that occurred during the first half of the 20th century, World War I (1914–1918) and World War II (1939–1945). However, some historians have also characterized other global conflicts as world wars, such as the Nine Years War, the War of the Spanish Succession, the Seven Years War, the French Revolutionary and Napoleonic Wars, the Cold War, and the War on Terror.