A gasket is a mechanical seal between two objects, usually used to prevent leakage from pressure, corrosion, and the natural thermal expansion and contraction of piping between the two objects. Since machined surfaces cannot be perfect, irregularities can be filled with gaskets. Gaskets are usually made of sheet materials such as gasket paper, rubber, silicone rubber, metal, cork, felt, neoprene, nitrile rubber, fiberglass or plastic polymers (e.g. PTFE). Gaskets for specific applications may contain asbestos.

A washer is a thin plate (usually round) with a loophole (usually in the center) usually used to distribute the load of a threaded fastener. Other uses are as spacers, springs (Belleville shims, wave shims), wear pads, predisplay devices, and locking devices. Rubber gaskets are also used in faucets (valves) to shut off the flow of liquids or gases. Rubber or silicon gaskets can also be used to minimize fan vibration. Usually the outer diameter of the gasket is about twice the inner diameter .

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