Arrhythmia is a medical term that refers to an irregular or abnormal heart rhythm. It occurs when the electrical impulses that control the heart's contractions do not function properly, resulting in: 

• Tachycardia: A rapid heart rate
• Bradycardia: A slow heart rate
• Atrial fibrillation: A quivering or fluttering of the upper chambers of the heart
• Ventricular fibrillation: A life-threatening condition where the heart's lower chambers contract irregularly and chaotically

Arrhythmias can be caused by various factors, including heart disease, electrolyte imbalances, certain medications, and inherited conditions. They can range from mild to life-threatening and require medical attention for diagnosis and treatment.