Final answer:
An ectopic focus or ectopic pacemaker is the term for an abnormal condition where cells that aren't normally pacemakers depolarize spontaneously. It can be caused by various factors and may lead to arrhythmias or fibrillation if chronic.
Step-by-step explanation:
The abnormal condition where cells that aren't normally pacemakers depolarize spontaneously is known as an ectopic focus or ectopic pacemaker. This occurs when an area of the heart other than the SA node initiates an impulse and leads to premature contractions. Various factors such as localized ischemia, certain drugs, elevated stimulation by the autonomic nervous system, or specific disease conditions can stimulate an ectopic focus. While occasional occurrences are generally non-life-threatening, chronic ectopic foci can lead to arrhythmias or fibrillation.