HEPATICO*CHOLANGIO*CHOLECYST*ENTERO*STOMIES = a surgical creation of a connection between the gall bladder and a hepatic duct and between the intestine and the gall bladder.
This is the longest word in Gould's Medical Dictionary.
TETRA*METHYL*DIAMINO*BENZHYDRYL*PHOSPHINOUS = a type of acid.
This is the longest chemical term in the Oxford English Dictionary (2nd Ed.). It does not have its own entry but appears under a citation for another word.
PNEUMONO*ULTRA*MICRO*SCOPIC*SILICO*VOLCANO*CONIOSIS (also spelled PNEUMONO*ULTRA*MICRO*SCOPIC*SILICO*VOLCANO*KONIOSIS) = a lung disease caused by breathing in particles of siliceous volcanic dust.