World War I (often abbreviated as WWI or WW1), also known as the First World War, the Great War, the Seminal Catastrophe, and initially in North America as the European War, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918.

Contemporaneously described as "the war to end all wars", it led to the mobilization of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history. It is also one of the deadliest conflicts in history, with an estimated nine million combatants and seven million civilian deaths directly resulting from the war, while resulting genocides and the 1918 influenza pandemic caused another 50 to 100 million deaths worldwide.

The term "First World War" was first used in September 1914 by German biologist and philosopher Ernst Haeckel. They claimed that "there is no doubt that the course and character of the feared 'European War' ... will become the first world war in the full sense of the word," citing a wire service report in The Indianapolis Star on 20 September 1914.

Before World War II, the events of 1914–1918 were generally known as the Great War or simply the World War. In October 1914, the Canadian magazine Maclean wrote, "Some wars name themselves; this is the Great War." Contemporary Europeans also referred to it as "the war to end war" or "the war to end all wars" due to their perception of its then-unparalleled scale and devastation. After World War II began in 1939, the terms became more standard, with British Empire historians, including Canadians, favoring "The First World War" and Americans "World War I".