A hydro static seal is a type of mechanical seal

Seal (mechanical)

A mechanical seal is a device which helps join systems or mechanisms together by preventing leakage , containing pressure, or excluding contamination...

. It is a non-contacting seal seals by chocking the flow from one side of the seal to the other. A film is developed between the two sealing surfaces which prevent the two surfaces from touching. They can be used for high speed applications and can have a much longer life than contacting seals as they ware at a much lower rate.