Embolism: The obstruction of a blood vessel by a foreign substance or a blood clot blocking the vessel. Something travels through the bloodstream, lodges in a vessel and plugs it.

Foreign substances that can cause embolism include an air bubble, amniotic fluid, a globule of fat, a clump of bacteria, chemicals (such as talc), and drugs (mainly illicit ones).

Blood clots are the most common cause of embolism. A pulmonary embolus is a blood clot that has been carried through the blood into the pulmonary artery (the main blood vessel from the heart to the lung) or one of its branches, plugging that vessel.

The term "embolus" refers to the plug itself obstructing the blood vessel while "embolism" refers to the process by which this happens.

The word "embolus" comes from the Greek "embolos" meaning a wedge or plug. "Embolos" was derived from "en" (in) + "ballein" (to throw) so an embolus is something thrown in.