Source : Webster's Revised Unabridged Dictionary (1913)

Watermelon \Wa"ter*mel`on\, n. (Bot.)
   The very large ovoid or roundish fruit of a cucurbitaceous
   plant ({Citrullus vulgaris}) of many varieties; also, the
   plant itself. The fruit sometimes weighs many pounds; its
   pulp is usually pink in color, and full of a sweet watery
   juice. It is a native of tropical Africa, but is now
   cultivated in many countries. See Illust. of {Melon}.

Source : WordNet®

watermelon
     n 1: an African melon [syn: {watermelon vine}, {Citrullus
          vulgaris}]
     2: large oblong or roundish melon with a hard green rind and
        sweet watery red or occasionally yellowish pulp