A deep vein is a vein that is deep in the body. This contrasts with superficial veins that are close to the body's surface.
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.
Deep veins are these blood vessels which are embedded far into the flesh, and if needing operation, it can be difficult.