What is an embolus?

In the bloodstream: Embolus: 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.

See below: Generally it is a blood clot/organized, that forms in one location (generally the heart) and travels to another location in the body.

Blockage of blood: Embolism is something that blocks proper flow of blood in the body. An embolism is usually a blood clot that can forms in your veins or arteries that can lodge in dangerous places in your circulatory system. Venous embolisms can cause catastrophic breathing problems. Air, fat and a few other things can also be a type of embolism that blocks blood flow.