Sentence:  An extreme version of vorticity is a <WSD>"vortex</WSD>". The <WSD>vortex</WSD> is a spinning, cyclonic mass of fluid, which can be observed in the rotation of water going down a drain, as well as in smoke rings, tornados and hurricanes. 
Meaning of the word marked with <WSD> tags:  A whirlwind, whirlpool, or similarly moving matter in the form of a spiral or column.