A blood clot that travels through the bloodstream is called an embolus. More precisely, a traveling clot is a thromboembolus, but embolus is the standard quiz answer for the general concept.
A clot usually begins as a thrombus, meaning a stationary blood clot attached to the wall of a blood vessel. If part of it breaks loose, blood flow carries the detached material elsewhere, where it may block a narrower vessel. That traveling mass is then an embolus.
The word embolus comes from the Greek word embolos, meaning “wedge” or “plug.” German pathologist Rudolf Virchow introduced the term in the 19th century during foundational work on thrombosis and blood clots. Although most emboli are clots, embolic material can also include fat droplets, air bubbles, cholesterol plaque, or foreign substances.
A pulmonary embolism occurs when an embolus blocks an artery in the lungs. The related terms are easy to mix up: thrombus means a clot that remains in place, embolus means material that travels, and embolism describes the blockage or event caused when it lodges.