What is Sich?

Legal Definition
A sich (Ukrainian: Січ) is the administrative and military centre for Cossacks and especially the Zaporozhian Cossacks. It is derived from the Ukrainian word siktý, "to chop", meaning to clear a forest for an encampment, or to build a fortification with the trees that have been chopped down.

The Zaporizhian Sich was the fortified capital of the Zaporozhian Cossacks in the 16th to 18th century area of Ukraine, and it was on the Dnieper River. The Sich Rada was the highest branch of government in the Zaporizhian Host or army of the Zaporozhian Cossacks. The Danubian Sich was the fortified settlement of the Zaporozhian Cossacks when they later settled in the Danube Delta.
-- Wikipedia
Legal Definition
A little current of water, which is dry in summer; a water furrow or gutter. Cowell.
-- Black's Law Dictionary