Real Dictionary

watermelon
variable noun: A watermelon is a large round fruit with green skin, pink flesh, and black seeds.

watermelon in American English
a large, round or oblong, edible fruit with a hard, green rind and sweet, juicy, pink or red pulp containing many seeds
noun: the large, roundish or elongated fruit of a trailing vine, Citrullus lanata, of the gourd family, having a hard, green rind and a sweet, juicy, usually pink or red pulp

watermelon in British English
noun: an African melon, Citrullus vulgaris, widely cultivated for its large edible fruit