Often abbreviated AVNRT. Here is the definition from arrythmia.org:
AV nodal reentrant tachycardia is the most common form of paroxysmal supraventricular tachycardia or PSVT. Patients afflicted with this arrhythmia do not usually have other structural problems with their heart. The arrhythmia originates in the tissues near the AV node, the electrical structure that transmits impulses between the upper and lower chambers of the heart. Susceptible individuals will have two pathways that can channel impulses to and from the AV node. Under the right conditions, usually following a premature beat, these pathways can form an electrical circuit. An impulse will revolve around this circuit and each revolution will lead to impulse propagation to the ventricles, and thus a rapid heart beat.
Frequently treated with calcium channel blockers, although some patients can control it with deep breathing, meditation, and biofeedback.