1. An abnormal particle; such as, an air bubble or part of a clot, circulating in the blood.
2. 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/or some foreign material; such as, air.

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.

3. Etymology: 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".