A microscopic organism (plant or animal).
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mi·cro·or·gan·ism -'ȯr-gə-.niz-əm n an organism of microscopic or ultramicroscopic size
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(microbe)
n.
any organism too small to be visible to the naked eye. Microorganisms include bacteria, some fungus, mycoplasma, protozoa, rickettsiae, and virus.
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mi·cro·or·gan·ism (mi″kro-orґgən-iz-əm) [micro- + organism] a microscopic organism; those of medical interest include bacteria, fungi, and protozoa. Viruses are often classified as microorganisms, although they are sometimes excluded because they are not cellular and they are unable to replicate without a host cell. microorganic, microorganismal adjMedical dictionary. 2011.