The "Open Door" was the name given to the United States' fundamental policy toward China for much of the 19th century and the first half of the 20th, up until World War II. Under the "Open Door" policy, America made no territorial demands of China nor did it seek any sphere of influence in the Chinese mainland like other powers, but it always insisted that China remain an "open door" to all nations (i.e. especially the US) - that no nation receive special priviledges not extended to all others, that all Chinese "treaty ports" be equally open to all nations, and that no nation be unfairly discriminated against with special tariffs. Because the US almost never intervened militarily in China, yet demanded all the same priveledges granted those powers that did use force to extract new concessions from the Chinese, the US Open Door policy came to be known as "jackal diplomacy," as the US "jackal" benefited from the efforts of the British "lion."
The Open Door policy had its origins in the "most favored nation" clauses typically included in the treaties China made with the Western powers including the US following the Opium War, and the traditional Chinese practice of treating all foreigners exactly the same (essentially because all foreigners were equally barbarians to the Chinese). Long unofficial US policy, the US Open Door stance moved toward official international recognition when several European powers sought to receive special unequal priveledges from China in the 1890s, prompting McKinley's Secretary of State John Hay to sent a round of missives to all the European powers in September of 1899, requesting them to respect equal rights for all nations under the treaties. The Open Door policy was later made official international doctrine during the nine-power Washington Disarmament Conference of 1921-1922.
As the 1930s progressed, the Japanese increasingly trampled on the international community's Open Door rights as they successively overran Manchuria, Inner Mongolia, and Northern China. By the end of World War II, the Open Door policy was irrelevant, because the new Peoples Republic of China would only deal with Communist Russia.