the part of a plant or tree that grows under the ground and gets water from the soil.——朗文当代大辞典

The persistent underground part of a plant (see sense 2) used for eating or (esp. formerly) as a source of medicine. Now esp.: the fleshy and enlarged root of any of numerous plants used as vegetables (e.g. a turnip or carrot); a root vegetable.——牛津当代大辞典  @en