World war
Historians like Richard F. Hamilton and Holger H. Herwig created a list of eight world wars, including the two generally agreed-upon world wars, the Seven Years' War, and five others: the Nine Years' War (1689–1697), the War of the Spanish Succession (1701–1714), the War of the Austrian Succession (1740–1748), the French Revolutionary Wars (1792–1802), and the Napoleonic Wars (1803–1815). British historian John Robert Seeley dubbed all of those wars between France and Great Britain (later the UK) between 1689 and 1815 (including the American Revolutionary War from 1775 to 1783) as the Second Hundred Years' War, echoing an earlier period of conflict between France and England known as the Hundred Years' War (1337–1453). Some writers have referred to the American Revolutionary War alone as a world war. Others (like William R. Thompson or Chase-Dunn and Sokolovsky) also wants to include the Italian Wars and Dutch wars [Dutch-Spanish and Anglo-Dutch Wars] as part of Global Wars, while clasificating WW1 and WW2 as the Global German Wars, and the Coalition Wars with Wars of Louis XIV as the 2nd and 1st Global French Wars. However, other historians prefer to see all of those conflicts as "Hegemonic Wars" or "General Wars", been inter-regional wars on the grand scale, but not worldly.
Other historians suggest even earlier conflicts to be world wars. For example, Russian ethnologist L. N. Gumilyov called the Byzantine–Sasanian War of 602–628 "the World War of the 7th century" because it evolved into a war between the fourfold alliance of the Chinese Empire, the Western Turkic Khaganate, the Khazars, and the Byzantine Empire against a triple union of the Sasanian Empire, the Avars, and the Eastern Turkic Khaganate, with proxy conflicts in Afro-Eurasia (like the Aksumite–Persian wars) and across the Old World.
Others consider that the Ottoman–Portuguese confrontations and Ottoman–Habsburg wars can be considered as world conflicts, prototypes of the "Great Game" in Eurasia and the Scramble for Africa, but between two main power-projecting and religious blocs, that being the Ottomans, as holders of the Muslim Caliph title, and the Habsburgs, as emperors of Christendom. Also has been argued that a Catholic-Muslim Global War hadn't started with the clash between Charles V and Suleiman the Magnificent aspirations of Universal powers, but before withe the Spanish-Ottoman Conflicts in late Reconquista, due to Catholic Monarchs of Spain's attempt to attack Turks and Mamluks (supporters of Emirate of Granada) in Asia from the East Indies, while also attacking them in Mediterranean Europe and North Africa (aspiring to lead a Global Crusade at the end, that started their grandson).
However, the Americas and Oceania were not involved in those conflicts, in which case, other historians consider the Thirty Years' War and Eighty Years' War (specially Iberian–Dutch War) as the first global conflict, pitting the Spanish and Portuguese Empires against the French, Dutch, and British Empires and their allies (mostly Protestants, like Danish and Swedish oversea expeditions) across the five continents.
Another possible example is the Second Congo War (1998–2003) even though it was only waged on one continent. It involved nine nations and led to ongoing low-intensity warfare despite an official peace and the first democratic elections in 2006. It has been referred to as "Africa's World War".

See also: Interwar period
List of largest empires
List of military conflicts spanning multiple wars
List of ongoing armed conflicts
Military history

References: 

Bibliography: Shapiro, Fred R.; Epstein, Joseph (2006). The Yale Book of Quotations. Yale University Press. ISBN 978-0-300-10798-2.
Willmott, H. P. (2003). World War I. Dorling Kindersley. ISBN 978-0-7894-9627-0. OCLC 52541937.

External links: This is the Fourth World War, an interview with philosopher Jean Baudrillard