Normally referring to a pipe fitting that is used to reduce the diameter of the suction piping to fit the pump intake.
Reducers are available in both eccentric and concentric designs. The eccentric reducer is shown on the left and the concentric reducer on the right
Be careful to install the eccentric reducer with the straight side up to prevent trapping air at the pump suction.
Concentric reducers frequently trap air when used at the suction of the pump.