Meaning

noun the alliance of nations that fought the Axis in World War II and which (with subsequent additions) signed the charter of the United Nations in 1945

noun in World War I the alliance of Great Britain and France and Russia and all the other nations that became allied with them in opposing the Central Powers

noun an alliance of nations joining together to fight a common enemy

originProprialisation of allies, the plural of ally.

Etymology from Wiktionary, CC BY-SA 3.0