The German term Anschluss translates to attachment or inclusion. It was used to refer to the idea of the inclusion of Austria in a "Greater Germany". The idea of the incorporation of Austria into Germany arose a few times including in 1919 following the First World War and the collapse of the Austro-Hungarian Empire.
However in March 13, 1938 the idea of Anschluss became a reality when Austria was annexed by Nazi Germany. The day before German troops had arrived under the orders of Adolf Hitler the German Chancellor who had himself been born and raised in Austria. The union lasted until 1945 and the end of World War II when Austria became a republic again.