noun
: any of a system of three veins which drain the thoracic wall and much of the abdominal wall and which form a collateral circulation when either the inferior or superior vena cava is obstructed ; especially : a vein that receives blood from the right half of the thoracic and abdominal walls, ascends along the right side of the vertebral column, and empties into the superior vena cava — see hemiazygous vein