The 1938 union of Austria with Germany by the Nazis in defiance of the Treaty of Versailles. Literally "junction" or "connection".

Also the sound made by a huge stack of unopened mail and bills sliding onto the floor from the dining room table or sideboard.

The incorporation of Austria into the German empire during WW2.
The event - immediately making Austria very unsafe for Jews - took place the 13th of March 1938 - and just six months after, all Austrians more or less joined in for the Night of Broken Glass.
The word is a German noun and means "joining".