An Implantable Cardioverter-Defibrillator (ICD) is a device similar to a pacemaker but with the added ability to deliver an electrical shock (defibrillation) to correct dangerously fast heart rhythms (tachycardia or fibrillation). It's implanted under the skin and continuously monitors the heart, providing life-saving therapy for patients at high risk of sudden cardiac arrest due to severe arrhythmias.