Definition of World War I noun from the Oxford Advanced American Dictionary

 

World War I

 noun
noun
NAmE//ˌwərld wɔr ˈwʌn//
 
(also the First World War)
 
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a war that was fought from 1914 to 1918, mainly between France, the U.K., Russia, and the U.S. on one side, and Germany, Austria-Hungary, and Turkey on the other side