Merriam-Webster Collegiate Dictionary - citron

 noun  Etymology: Middle French, modification of Latin citrus  Date: 1530  1.  a. a citrus fruit resembling a lemon but larger with little pulp and a very thick rind  b. a small shrubby tree (Citrus medica) that produces ~s and is cultivated in tropical regions  c. the preserved rind of the ~ used especially in cakes and puddings  2. a small hard-fleshed watermelon used especially in pickles and preserves