A concentration camp is a place where people are kept against their will. It is distinguished from a prison because it functions outside of a judicial system, and inmates are not convicted of any crime by a transparent judicial process. During the Nazi period, concentration camps were used across German-occupied lands to arrest and imprison those that were either political opponents or seen as ideological enemies. During the Third Reich, there were more than 44,000 camps across Europe, each serving different functions, including as death/extermination camps, labour camps, internment camps and transit camps. The first Nazi concentration camp opened in 1933 for political opponents.