A fitting that unites different types or materials of pipe, such as ABS to cast-iron pipe.
Merriam-Webster Online Dictionary
adapter (noun)1.
one that - adapts
2.
a) a device for connecting two parts (as of different diameters) of an apparatus
b) an attachment for apparatus for uses not originally intended - adapting
adapter (Wikipedia)
This mains power plug travel adapter allows British plugs to be connected to American or Australian sockets.
An (electrical) adapter or adaptor is a device that converts attributes of one electrical device or system to those of an otherwise incompatible device or system. Some modify power or signal attributes, while others merely adapt the physical form of one electrical connector to another.