Abrasive waterjet machining involves the impact of micro-particles at high or ultrahigh velocities. The material removal mechanism for ductile materials has been popularly accepted for over a half century as cutting wear and deformation wear caused by the component of impact force parallel and perpendicular to the target surface respectively. However, this definition of erosion mechanisms does not give an insight into the erosion process, but describes a surface phenomenon of the event. A computational study has been undertaken to reveal the underlying mechanisms of the material removal process. Based on the findings, the impact erosion mechanisms are re-defined as material destruction through (a) failures induced by inertia, (b) failure induced by elongation, and (c) failure induced by adiabatic shear bending. This new definition appears to better represent the physical process of material deformation and removal by loose micro-particle impacts at high and ultrahigh velocities.