One China Policy
The One-China policy refers to the policy or view that there is only one state called "China", despite the existence of two governments that claim to be "China". As a policy, this means that countries seeking diplomatic relations with the People's Republic of China (PRC, Mainland China) must break official relations with the Republic of China (ROC, Taiwan) and vice versa.

The One China policy is also different from the "One China principle", which is the principle that insists both Taiwan and mainland China are inalienable parts of a single "China".

China foreign Ministry response to Trump’s statement
China’s Foreign Ministry on Monday warned that any change in the U.S.’s one-China policy will impair ties between Beijing and Washington.
“Upholding the ‘one China’ principle is the political basis for developing China-U.S. ties. 

Trump’s statement
In an interview on Fox News by President-elect Donald Trump, where he questioned the U.S.’s persistence with the one-China policy, unless there were concessions from Beijing on trade and other issues.
He also blamed China for devaluing its currency, and not cooperating with Washington on North Korea and the South China Sea.
The one-China policy underscores recognition of China’s sovereignty over Taiwan — a position that has been held by the U.S. since 1979.