nanobacteriumEnglishOxford New American Dictionarynanobacterium /ˌnanōbakˈtirēəm, ˌnænoʊbækˈtɪriəm / ▸ noun (plural nanobacteria /-ˈti(ə)rēə/) a microorganism about a tenth the size of the smallest normal bacteria, claimed to have been discovered in living tissue and in rock.