Internment means putting a person in prison or other kind of detention, generally in wartime. During World War II, the American government put Japanese-Americans in internment camps, fearing they might be loyal to Japan.
Internment usually doesn’t involve a trial, so you're being held because someone thinks you might be dangerous, but there’s no proof. The internment of Japanese-Americans during World War II is now widely considered to have been a terrible mistake, in that the citizens who were detained — some for as long as four years — were not traitors, but loyal Americans, and their internment caused them considerable emotional and economic hardship. Internment comes from the Latin internus, “inward.”