Narrow complex refers to a QRS duration of less than 120 milliseconds on an electrocardiogram, indicating normal ventricular depolarisation through the His-Purkinje system. It is clinically important for distinguishing supraventricular tachycardias from ventricular arrhythmias, guiding appropriate management. A key association is atrial fibrillation with rapid ventricular response presenting as a narrow complex tachycardia.