Blood clot: A mass of blood that forms when blood platelets, proteins, and cells stick together. When a blood clot is attached to the wall of a blood vessel, it is called a thrombus. When it moves through the bloodstream and blocks the flow of blood in another part of the body, it is called an embolus. Signs and symptoms of a pulmonary embolism include anxiety, sudden dyspnea, chest discomfort, tachycardia or tachypnea, low fever, pleural friction rub, and wheezing. Pulmonary embolism is a medical emergency. Patients with multiple myeloma are at increased risk for a pulmonary embolism because of the disease and treatments for multiple myeloma.