The medical term embolus arises from the Greek [έμβολο] (pronounced émvolo) meaning "a plug", or "a plunger". This Greek term was later adopted in Latin [embolus] and is used in this unchanged form today. The plural for embolus is [emboli].
In medicine, the term embolus usually refers to a free blood clot that travels down the bloodstream. When in the veins, emboli will travel easily to and through the heart. This is because veins increase in diameter towards the heart. The opposite happens in arteries. Free blood clots (emboli) that passed through the heart with no problem now enter the pulmonary arteries whose branches get smaller and smaller until the emboli plug the arterioles, and now the patient has a pulmonary embolism.
When thrombi are generated in the heart, they are usually generated in the left atrial appendage in cases of arrhythmia or atrial fibrillation. If these thrombi embolize, that is, they become free, these now emboli will continue downstream in the arteries and progressively smaller arterioles until they are bigger that the vessel and plug it, cutting off blood supply. This condition can cause an infarction, also known as a stroke.
The term embolus can also refer to liquids. fat, or gases that enter the blood stream and are not diluted.
The first use of this term in modern medicine was by Virchow in 1846 in his paper "On the Occlusion of the Pulmonary Arteries"
The root term for this word is [-embol-]. Examples of its use are:
• Embolism: The suffix [-ism] means "behavior" or "pathology".
• Thromboembolism: A combination of root terms; the root term [-thromb-] means "fixed clot" and [-embol] means "a free clot". A condition or presence of both trombi and emboli.
Sources:
1. "The Origin of Medical Terms" Skinner, HA 1970 Hafner Publishing Co.
2. "Medical Meanings - A Glossary of Word Origins" Haubrich, WD. ACP Philadelphia