The Third Reich was a common name for Germany under the Nazi rule between 1933 and 1945. The German word "Reich" best translates as "empire." The "First Reich" was the Germanic empire established under Charlemagne that became the Holy Roman Empire, the "Second Reich" was the unified German empire that lasted between 1871 and 1918. Hitler, in his dreams of dominating Europe, frequently spoke of establishing a "thousand year Reich."
When Hitler came to power in 1933, he quickly brought all aspects of German life under the control of the Nazi Party. The Weimar Republic was officially disbanded and the Deutsches Reich instituted. Mere weeks after becoming Chancellor, Hitler secretly ordered the burning of the Reichstag building, blaming the incident on communist agitators and using it as an excuse to suppress civil liberties throughout Germany. The most obvious victims of Nazi policies in the Third Reich were Jews. The Nuremburg Laws of 1935 stripped all Jewish people of their rights, beginning the descent into the Holocaust.
Hitler also quickly began violating the Versailles Treaty in order to rebuild the military capacity of the Third Reich. Military build up and persecution of Jews and minorities characterized the Third Reich during the 1930s, culminating in the 1939 invasion of Poland. At its height in 1942, the Third Reich encompassed nearly all of Europe.