SPECIAL ORDER ITEM. Review Returns Policy before purchase. An adapter is a general plumbing fitting used to connect 2 different types of pipe, sizes, or connection methods within a piping system. It acts as a transition piece, allowing otherwise incompatible components to be joined together securely. Adapters are used across many plumbing materials and systems, including PEX, copper, CPVC, PVC, stainless steel, and threaded pipe systems. Because of this, they come in many configurations depending on the application.