A vein is a blood vessel related to the circulatory system. Its main purpose is to transport blood back to the heart. In the world of anatomy, there are several other words you can use to describe a vein. One common synonym that is frequently used is blood vessel. Another synonym is venous tissue. Other words that you might use to describe a vein in a more metaphorical sense include stream, channel, or pipeline. Other synonyms for vein that relate more specifically to the appearance or function of veins include vessel, capillary, or artery. Regardless of which word you choose to use, veins are an essential component of the human body's circulatory system.