Final answer:
Cardiac arrest occurs when the heart stops pumping blood. Witnessed cardiac arrest is when someone sees the person collapse, while unwitnessed cardiac arrest is when someone finds the person unresponsive.
Step-by-step explanation:
Cardiac arrest occurs when the heart no longer pumps blood or pumps blood so poorly that vital organs can no longer function. This is a medical emergency requiring immediate intervention.
Witnessed cardiac arrest refers to a situation where someone sees the person collapse or the arrest happen, while unwitnessed cardiac arrest refers to a situation where someone finds the person unresponsive without any knowledge of the arrest itself.