An embolism occurs when an embolus (or emboli) becomes lodged into a blood vessel. When this occurs, it can cause a partial or full blockage of the blood vessel.
An embolism is a severe form of blood clot. It can be dangerous if it disrupts flow to a major organ such as the brain, heart, or lungs, since it can starve these vital organs of oxygen (via a reduced blood flow).
If the embolism is caused by a piece of a blood clot, this is called a thromboembolism.