Final answer:
An ectopic focus or ectopic pacemaker is a beat originating from a location outside of the SA node in the heart. It can be caused by various factors and may lead to arrhythmias or fibrillation if chronic.
Step-by-step explanation:
An ectopic focus or ectopic pacemaker refers to a beat arising from a location outside of the sinoatrial (SA) node, the heart's natural pacemaker. This can occur in various parts of the heart, such as the atria or ventricles, due to factors like localized ischemia, certain drugs, or autonomic nervous system stimulation. While occasional ectopic beats are typically transient and non-life threatening, a chronic condition can lead to arrhythmias or fibrillation.