The Lost Generation was the social generational cohort in the Western world that was in early adulthood during World War I. The generation is generally defined as people born from 1883 to 1900. The term is also particularly used to refer to a group of American expatriate writers living in Paris during the 1920s. Gertrude Stein is credited with coining the term, and it was subsequently popularised by Ernest Hemingway, who used it in the epigraph for his 1926 novel The Sun Also Rises: "You are all a lost generation." "Lost" in this context refers to the "disoriented, wandering, directionless" spirit of many of the war's survivors in the early postwar period.

In the wake of the Industrial Revolution, Western members of the Lost Generation grew up in societies which were more literate, consumerist and media-saturated than ever before, but which also tended to maintain strictly conservative social values. Young men of the cohort were mobilized on a mass scale for the First World War, a conflict that was often seen as the defining moment of their age group's lifespan. Young women also contributed to and were affected by the War, and in its aftermath gained greater freedoms politically and in other areas of life. The Lost Generation was also heavily vulnerable to the Spanish flu pandemic and became the driving force behind many cultural changes, particularly in major cities during what became known as the Roaring Twenties.

Later, they experienced the economic effects of the Great Depression and often saw their own sons leave for the battlefields of the Second World War. In the developed world, they tended to reach retirement and average life expectancy during the decades after the conflict, but some significantly outlived the norm. The last surviving person who was known to have been born during the 19th century was Nabi Tajima, who died in 2018.