n Any plant of the genus Rheum, especially Rheum rharbarbarum, having large leaf|leaves and long green or reddish acidic leafstalks, that are edible, in particular when cooked (although the leaves are mildly poisonous).
n The dried rhizome and roots of Rheum palmatum or Rheum officinale, from China, used as a laxative and purgative.
n (military) An RAF World War II code name for operations by aircraft (fighters and fighter bombers) seeking opportunity targets.