A gasket is a mechanical seal that is used to seal two or more surfaces that are in touch with each other. During compression, the seal prevents leaking into or from the joint components. For piping, the gaskets are the sealing materials used to form static seals between flanges.

Flange gaskets are designed to seal the space between two sections of pipe. Most pipes, of course, are joined at a flanged surface. The surface where two pipes join isn't smooth or otherwise flat. Rather, it's flanged, meaning it flares out.