Mast
From RC Helicopter Wiki
The main mast or main shaft is the shaft to which the main rotor is mounted. It carries torque from the main gear to the main rotor, and transmits load from the helicopter to the main rotor.
It will very often bend in a crash. You can check this by removing it from the aircraft and rolling it on a sheet of glass, or by using a dial indicator, or using permanent marker as a poor man's dial indicator.