A pump varies output flow. For all cross-sections, which are vertical to the axis of rotation between axial outer positions for the cross-sectional areas of flow at the periphery of the impeller, the inner wall of the pump housing forms approximately circular profiles. The approximately circular profiles are mainly concentric and have a continuously increasing radius from one toward the other one of the axial outer positions. A tongue, which truncates the outlet or diffuser of the pump from an annulus of the pump housing, does not contact the circular profiles between the axial outer positions.