The Collaborative International Dictionary of English v.0.48:

Melon \Mel"on\ (m[e^]l"[u^]n), n. [F., fr. L. melo, for melopepo
   an apple-shaped melon, Gr. mhlope`pwn; mh^lon apple + pe`pwn
   a species of large melon; cf. L. malum apple. Cf.
   Marmalade.]
   [1913 Webster]
   1. (Bot.) The juicy fruit of certain cucurbitaceous plants,
      as the muskmelon, watermelon, and citron melon; also, the
      plant that produces the fruit.
      [1913 Webster]

   2. (Zool.) A large, ornamental, marine, univalve shell of the
      genus Melo.
      [1913 Webster]

   Melon beetle (Zool.), a small leaf beetle (Diabrotiea
      vittata), which damages the leaves of melon vines.

   Melon cactus, Melon thistle.
      (a) (Bot.) A genus of cactaceous plants (Melocactus)
          having a fleshy and usually globose stem with the
          surface divided into spiny longitudinal ridges, and
          bearing at the top a prickly and woolly crown in which
          the small pink flowers are half concealed. Melocactus
          communis, from the West Indies, is often cultivated,
          and sometimes called Turk's cap.
      (b) The related genus Mamillaria, in which the stem is
          tubercled rather than ribbed, and the flowers
          sometimes large. See Illust. under Cactus.
          [1913 Webster]

WordNet (r) 3.0 (2006):

melon
    n 1: any of numerous fruits of the gourd family having a hard
         rind and sweet juicy flesh
    2: any of various fruit of cucurbitaceous vines including:
       muskmelons; watermelons; cantaloupes; cucumbers [syn:
       melon, melon vine]