Vein: A vein is a blood vessel that drains blood back to the heart. They are part of the cardiovascular system. It has thin, flexible walls when compared to an artery. Some veins have valves in them to prevent blood from running away from the heart again, such as in the legs. Veins are often named after the tissue, organ or area that they drain – but some have traditional anatomical names. For example, the jugular veins in the neck, which drain the brain and other parts of the head, are named for the Latin term for the neck – ‘jugularis’. Veins may develop clots in them, which can cause the area they drain to swell dangerously with blood.