Deep vein is a term used to describe a vein that is deep in the body. It is used to differentiate deep veins from veins which are close to the surface, also known as superficial veins.
Deep veins are almost always beside an artery with the same name (e.g. the femoral vein is beside the femoral artery). Collectively, they carry the vast majority of the blood. Occlusion of a deep vein can be life threatening and is most often caused by thrombosis. Occlusion of a deep vein by thrombosis is called deep vein thrombosis.