A detailed hierarchal nomenclature of arrhythmias is offered with definition of its applications to diagnosis and complications. The conceptual and organizational approach to discussion of arrhythmias employs the following sequence: location – mechanism – aetiology – duration. The classification of arrhythmias is heuristically divided into an anatomical hierarchy: atrial, junctional, ventricular, or atrioventricular. Mechanisms are most simplistically classified as either reentrant, such as macro-reentrant atrial tachycardia, previously described as atrial flutter, or focal, such as automatic or micro-reentrant tachycardia, for example, junctional ectopic tachycardia. The aetiology of arrhythmias can be either iatrogenic, such as postsurgical, or non-iatrogenic, such as genetic or congenital, and in many cases is multi-factorial. Assigning an aetiology to an arrhythmia is distinct from understanding the mechanism of the arrhythmia, yet assignment of a possible aetiology of an arrhythmia may have important therapeutic implications in certain clinical settings. For example, postoperative atrial arrhythmias in patients after cardiac transplantation may be harbingers of rejection or consequent to remediable imbalances of electrolytes. The duration, frequency of, and time to occurrence of arrhythmia are temporal measures that further refine arrhythmia definition, and may offer insight into ascription of aetiology. Finally, arrhythmias do not occur in a void, but interact with other organ systems. Arrhythmias not only can result from perturbations of other organ systems, such as renal failure, but can produce dysfunction in other organ systems due to haemodynamic compromise or embolic phenomena.