From WordNet (r) 3.0 (2006) [wn]:
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
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]