endosymbiosisPronunciation: /ˌɛndəʊˌsɪmbɪˈəʊsɪs, -bʌɪ-/Definition of endosymbiosis noun [mass noun] Biology symbiosis in which one of the symbiotic organisms lives inside the other. Derivatives endosymbiont noun endosymbiotic adjective endosymbiosis in other Oxford dictionaries Definition of endosymbiosis in the US English dictionary