"a guarded prison camp in which nonmilitary prisoners are held, esp one of those in Nazi Germany in which millions were exterminated"
"A place for assembling and confining political prisoners and enemies of a nation. Concentration camps are particularly associated with the rule of the Nazis in Germany, who used them to confine millions of Jews (see also Jews) as a group to be purged from the German nation. Communists, Gypsies, homosexuals, and other persons considered undesirable according to Nazi principles, or who opposed the government, were also placed in concentration camps and eventually executed in large groups. ( See Holocaust.)"
Particularly associated
The debate on the word is also well known, from https://en.m.wikipedia.org/wiki/Internment:
> As a result, some[who?] say that today the term "concentration camp" may be conflated with the concept of "extermination camp" and historians debate whether the term "concentration camp" or "internment camp" should be used in order to describe other examples of civilian internment, such as the United States government's internment of Japanese Americans during World War II,[4] the Australian government's immigrant detention facilities,[25][26] and South Korea's detention camps for "undesirables" in the 1970s and 1980s.[27]
> a place where large numbers of people, especially political prisoners or members of persecuted minorities, are deliberately imprisoned in a relatively small area with inadequate facilities, sometimes to provide forced labor or to await mass execution.
The most famous usage was by Germans and is heavily associated with extermination, but the British just wish to deprive the Boer’s of support and to put pressure on them into surrendering. Hence the term “concentration” rather than “extermination”. Many Boers did wind up dying due to inadequate sanitation and food, but the point wasn’t to kill them.
Whether the camps in Xinjiang are concentration camps or not remains to be seen. Definitely some of the awful work camps in China in the 70s met the threshold.