Wikipedia
Stammwürze
Stammwürze ( German for "original wort") is a type of measurement in brewing that is used to express the amount of extract (including sugars, dextrins, proteins, and minerals) in the pitched wort. It is closely related to gravity and is expressed as a weight percentage.
In order to be served at Oktoberfest, beer must be a minimum of 13.5% Stammwürze.