Originally (in full "polyarteritis acuta nodosa", "polyarteritis nodosa"): a systemic vasculitis characterized by focal inflammation and necrosis of the walls of medium-sized arteries, sometimes with aneurysm formation, often causing infarctions in organs supplied by affected vessels. Later also: any of several other forms of systemic vasculitis involving arteries or arterioles (and sometimes capillaries or venules); especially in "microscopic polyarteritis".