diphtheroidPronunciation: /ˈdɪfθərɔɪd, ˈdɪp-/Definition of diphtheroid noun Microbiology any bacterium of a genus that includes the diphtheria bacillus, especially one that does not cause disease. See corynebacterium. adjective [attributive] Medicine similar to diphtheria: diphtheroid endocarditisdiphtheroid in other Oxford dictionaries Definition of diphtheroid in the US English dictionary