Final answer:
An embolus is a thrombus that breaks free and enters the circulation. It can block a critical vessel and cause serious conditions like heart attack or stroke.
Step-by-step explanation:
An embolus is a thrombus that breaks free from the vessel wall and enters the circulation. It can be large enough to block a vessel critical to a major organ. When it becomes trapped, it is called an embolism. In the heart, brain, or lungs, an embolism can cause a heart attack, stroke, or pulmonary embolism.