An adjective meaning 'to do with veins' — the tubes that carry blood back toward your heart. It is a medical and scientific word: doctors, nurses and biology books use it, and ordinary people almost never do. In everyday talk you would just say 'in the vein' or 'in my veins'. You meet it in fixed pairs: venous blood (the blood in your veins, darker because it carries less oxygen), a venous catheter (a thin tube put into a vein), venous return (how blood gets back to the heart). It is often set against arterial, which means 'to do with arteries' — the tubes carrying blood away from the heart. There is also a plant use: a leaf can be described as venous if it has many visible lines, though botanists more often say veined or venose for that. Deep vein thrombosis, a serious blood clot, is sometimes written in hospital notes as 'deep venous thrombosis' — same thing.