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"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]