Dictionary.net

A cardiac arrhythmia caused by the interaction of two foci independently initiating cardiac impulses at different rates. Generally one of the foci is the sinoatrial node (the normal pacemaker) and the ectopic focus is usually in the ventricle, but can occur in the sinus and AV nodes, atrium and ventricle, and AV junction. Each focus - and thus each rhythm - is protected from the influence of the other.

The interval between the cardiac systole and diastole. [Gr.]