A mechanical seal is a part used to seal between a rotating part and a fixed part on rotating equipment. Ensuring the sealing between the pump shaft and the body would be an example of the most common use.

The basic components of mechanical seals are the rotating element and the fixed element. The main parts of the mechanical seal consist of a rotating face and a fixed face. There is a gap too small to allow fluid passage between these 2 components. This gap is lubricated by a thin film layer consisting of the fluid on the side where the pressure is high between the smooth surfaces. This layer both prevents friction between the two surfaces and provides sealing.