A euphemism for the ruthless removal or killing of an ethnic or religious group from areas that have been taken over by opposing forces.
Although the process has been engaged in since antiquity the term didn't enter the English language until the Balkan wars of the 1990s. The earliest citation of a phrase like it is from the New York Times, July 1982:
"The nationalists have a two-point platform, according to Becir Hoti, an executive secretary of the Communist Party of Kosovo, first to establish what they call an ethnically clean Albanian republic and then the merger with Albania to form a greater Albania."
The precise phrase appears first in the Washington Post, August 1991:
"The Croatian political and military leadership issued a statement Wednesday declaring that Serbia's 'aim is obviously the ethnic cleansing of the critical areas that are to be annexed to Serbia'."
See other phrases that were coined in the USA.