rhizoid Syllabification: (rhi·zoid)Pronunciation: /ˈrīˌzoid/Definition of rhizoid noun Botany a filamentous outgrowth or root hair on the underside of the thallus in some lower plants, especially mosses and liverworts, serving both to anchor the plant and (in terrestrial forms) to conduct water. Derivatives rhizoidalPronunciation: /rīˈzoidl/ adjective rhizoid in other Oxford dictionaries Definition of rhizoid in the British & World English dictionary