The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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