The non-moving part of a valve. Water flow is stopped when the moveable portion of the valve comes in contact with the valve seat.
The valve seat in an internal combustion gasoline or diesel engine is the surface against which an intake or an exhaust valve rests during the portion of the engine operating cycle when that valve is closed.
Surface against which a valve contacts. Usually, a valve at rest is against the inner seat, and a valve that is actuated is against the outer seat.