That would be the engine crankshaft. It is the major rotating assembly in the motor that the pistons and connecting rods are connected to. More than likely, if the motor has a bad crank, it probably has other bad parts.
This could include ;
the crankshaft bearings, damaged pistons/connecting rods, cylinder wall scoring/gouging and possible head/valve and or other valve train damage. The only way to know for sure is a complete tear down and inspection of the motor.