Emboli: Something that travels through the bloodstream, lodges in a blood vessel and blocks it. Examples of emboli are a detached blood clot, a clump of bacteria, and foreign material such as air.
Pulmonary emboli are blood clots that have 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.
Emboli is the plural of embolus, a word that comes from the Greek "embolos" meaning a wedge or plug. "Embolos" was derived from "en" (in) + "ballein" (to throw) so emboli are something thrown in.