A thrombus is a blood clot that forms within a blood vessel, obstructing blood flow. It can occur in veins or arteries and is composed of platelets, fibrin, and red blood cells. If a thrombus dislodges and travels through the bloodstream, it can lead to serious conditions such as a stroke or pulmonary embolism. The formation of a thrombus is often related to factors like injury to the blood vessel, slow blood flow, or increased blood clotting tendencies.