noun the root of several species of rheum, used much as a cathartic medicine.
2. rhubarb ·noun the name of several large perennial herbs of the genus rheum and order polygonaceae.
3. rhubarb ·noun the large and fleshy leafstalks of rheum rhaponticum and other species of the same genus. they are pleasantly acid, and are used in cookery. called also pieplant.