A<unk> occurs when electrical impulses from different sources act upon the same region of the heart at the same time.  If it acts upon the ventricular chambers, it is called a ventricular<unk>  wheres colliding currents in the atrial chambers produce atrial<unk> .
Fusion beats
Junctional rhythm
Ventricular escape beat
Poor R Wave Progression
Atopy
U wave alternans
Early repolarization
Sine wave pattern