So small as to be visible only with a microscope
protozoa are microscopic amoeba-like organisms
Made using an electron microscope; relating to or involving the use of an electron microscope.
Too small to be seen by an ordinary light microscope
Too small to be seen by an ordinary optical microscope
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.