Nazism, from German Nazi, as abbreviation of National Sozialismus, is a totalitarian ideology of the German NSDAP party, led by Adolf Hitler. The consequence of adopting this ideology was the World War II and the Holocaust.
Questions related to the NSDAP (National-Sozialismus Deutsche Arbeiterpartei), the political and economical system of Germany (3. Reich) between 1933 and 1945 and occupied countries should be marked with this tag. Similar movements in other countries having the similar programme (like Mosley's British Union of Fascists) do also apply.
History of neo-nazi movements can also be tagged with nazism.
Questions related to Adolf Hitler should bear the hitler tag.
For the time period of 1939 to 1945 consider using the ww2 tag.
For similar system, called fascism, in Italy, Spain and other countries use the fascism tag.