n. The fruit of the citron-tree, a variety of Citrus medica, distinguished from the lemon by the absence of an umbo at the summit and by its very thick rind.n. The citron-tree, Citrus medica.n. A round and nearly solid variety of the watermelon, Citrullus vulgaris, with white and almost flavorless flesh, sometimes used as a preserve.n. Same as citron-water.