What is an electrical shock delivered to the heart to correct abnormal heart rhythms called?

Defibrillation: If it's a potentially fatal heart rhythm (eg ventricular tachycardia or fibrillation), it's called defibrillation. In this case, a large amount of energy is delivered directly to the heart to stop it so that the internal pacemaker can re-activate it to a (hopefully) normal rhythm. In all cases, CPR (chest compressions) are required as well.

Cardioversion: If non-fatal (like atrial fibrillation), it's cardioversion in which a lower energy & specific electrical phase is given with the patient sedated but awake so that the rhythm can be converted to the normal, sinus rhythm.

Cardioversion-defib: An implantable aicd has cardioversion and defibrillation capabilities. However, usually, it delivers a defibrillatory shock when it detects a dangerous rhythm. Of course, the energy is much lower since less tissue needs to be penetrated (the electrical lead goes directly into the heart).