1520s, "place where an army lodges temporarily," from French camp, in this sense from Italian campo, from Latin campus "open field, level space," especially "open space for military exercise" (see campus).
The direct descendant of Latin campus in French is champ "a field." The Latin word had been taken up in early West Germanic as *kampo-z and appeared originally in Old English as camp "contest, battle, fight, war." This word was obsolete by mid-15c.
Transferred to non-military senses by 1550s. Meaning "body of adherents of a doctrine or cause" is from 1871. Camp-follower "one who follows an army without being officially connected to it," such as sutlers, washer-women, etc., first attested 1810. Camp-meeting "religious meeting for prayer, etc., held in an outdoor camp" is from 1809, American English, originally and especially in reference to Methodists. Camp-fever (1758) is any epidemic fever incident to life in a camp, especially typhus or typhoid. A camp-stool (1794) has a flexible seat and cross-legs and is made to be folded up and packed away when not in use.
1944, in reference to the Holocaust, probably translating German Todeslager; they also were known as extermination camps (German Vernichtungslager); historians usually count six of them: Auschwitz-Birkenau, Chełmno, Bełżec, Majdanek, Sobibór, Treblinka.