In plumbing, an adapter is generally a fitting that interfaces two different parts. The term commonly refers to:
*any fitting that connects pipes of different materials, including:
**expansion adapters which have a flexible section to absorb expansion or contraction from two dissimilar pipe materials
**mechanical joint (MJ) adapters for joining polyethylene pipe to another material
**bell adapters which are like mechanical joint adapters but contain a stainless steel backup ring to maintain a positive seal against the mating flange
**flange adapters which attach to a polyethylene pipe with butt fusion to stiffen a junction and allow another flanged pipe or fitting to be bolted on
*a fitting that connects pipes of different diameters, genders, or threads 
**adapter spools (also called crossover spools), used on oilfields and pressure control, have different diameters, pressure ratings or designs at each end
**adapters to convert NPT to BSP pipe threads are available
*a fitting that connects threaded and non-threaded pipe