The Declaration of Neutrality was a declaration by the Austrian Parliament declaring the country permanently neutral. It was enacted on October 26th 1955 as a constitutional act of parliament.
The declaration was a direct result of the allied occupation by the Soviet Union, the United States, the United Kingdom and France between 1945 and 1955. The Soviet Union would not have agreed to the signing of a State Treaty in May 1955 if Austria had not committed itself to declare its neutrality after the allied forces had left the country. The final foreign troops left Austria on October 25th 1955.
Since 1955, neutrality has become a deeply ingrained element of Austrian identity. Legally, Austrian neutrality is both a part of the Austrian constitution and of international law. There is some debate as to whether Austria could change this neutral status on its own account and whether Austria is still truly neutral, given it is a member of the EU.