Phlebitis: Inflammation of a vein. With phlebitis, there is infiltration of the walls of the vein and, usually, the formation of a clot (thrombus) in the vein (thrombophlebitis). Phlebitis in a leg, for example, causes the leg to swell with fluid (edema). Phlebitis can be superficial and not very serious, or it can be deep and carry the potential for dislodging blood clots to the lungs.
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