A world war was a large-scale war which affected the whole world. Six world wars were known to have occurred in Earth's history.
World War I was fought on Earth fought over four years, four months and four days (COMIC: The Weeping Angels of Mons) from 1914 to 1918. (TV: The War Games)
World War II began in Europe in September 1939, (PROSE: Timewyrm: Exodus, AUDIO: Neverland) and earlier in Asia in July 1937 after a period of unrest beginning in 1931. (PROSE: The Shadow of Weng-Chiang, Log 384) Worldwide hostilities formally ended in September 1945. (PROSE: Base of Operations)
Sometime after, World War III and then World War IV were fought. Indeed, Vicki Pallister noted that there had been four World Wars in London's history by her native time of the 25th century. St Paul's Cathedral had survived the four World Wars as well as the 22nd century Dalek invasion. (AUDIO: Frostfire)
According to one account, World War V was fought in the 49th century. (AUDIO: Singularity) According to another account, in the 37th century, the war was described by Cazkelf as "Earth's recent history". (COMIC: Art Attack)
World War VI took place in the year 5000. (TV: The Talons of Weng-Chiang, PROSE: Doctor Who and the Talons of Weng-Chiang, AUDIO: The Butcher of Brisbane)