protozoon n any of diverse minute acellular or unicellular organisms usually nonphotosynthetic Usage(s) Cause of the disease is a trypanosome, protozoon which under the microscope resembles an auger. The Anopheline mosquito bites a person and injects the malaria organism, a protozoon about one-fifth the size of a red blood cell. The cause of the disease is an oval protozoon with a short, slender tail. Synonym(s) protozoan