Definition of BIOREGION. : a region whose limits are naturally defined by topographic and biological features (as mountain ranges and ecosystems).— bio·re·gion·al \ˌbī-ō-ˈrēj-nəl, -ˈrē-jə-nəl\ adjective.ADVERTISEMENT. a region whose limits are naturally defined by topographic and biological features (as mountain ranges and ecosystems).