The adapter is the metal piece placed between the wet end and power end of an ANSI (American National Standards Institute) pump (#71). The adapter connects and aligns the power end of the ANSI pump to the wet end.
Adapters are available in either forged or cast metal. Forged is better.
A second adapter is sometimes used to connect and align the motor to the pump. This adapter is called a “C or D” frame adapter.