A blood clot (a clump of blood) that forms in a vein and causes inflammation is called thrombophlebitis. Symptoms include pain, tenderness, warmth, and redness along the length of a vein.
A clot may form either in a vein just under the surface of the skin (superficial thrombophlebitis) or in a deep vein (deep vein thrombosis, or DVT).
Thrombophlebitis can occur in any vein, but it is most common in the leg veins. Clots in a deep leg vein (deep vein thrombosis) are serious because a clot can break loose and travel through the bloodstream to the lung (pulmonary embolism).