It was sometimes also referred to as the "Thousand Year Reich", as it was intended by its founder, Adolf Hitler, to stand for 1000 years.
Hitler very self-consciously tried to align his movement with great successes of the past which would resonate within the German people, and he definitely intended that his creation would bring his name and image to the whole world for 1000 years, so both terms were used repeatedly to build this sense of history, strength and inevitability in the minds of all Germans.
Initially Hitler's plans seemed to be well on their way to fruition. At its height, the Third Reich controlled the greater part of Europe. However, due to the defeat by the Allied powers in World War II, the Third Reich lasted only 12 years, from 1933 through to 1945 (see Nazi Germany).
See also millennialism.