Watermelon refers to both the edible fruit and vine-like plant (Citrullus lanatus of the family Cucurbitaceae) of a climbing and trailing herb originally from southern Africa and one of the most common types of melon.

This flowering plant produces a special type of fruit known by botanists as a pepo, a type of egigynous berry or false berry (accessory fruit) that is derived from an inferior ovary and is characteristic of the Cucurbitaceae, a family that also includes cucumbers, squashes (such as pumpkins), luffas, and muskmelons (such as cantaloupe). In species of Cucurbitaceae, other parts of the flower (including the basal parts of the sepals, petals, and stamens) can ripen along with the ovary, forming the false berry.
The watermelon has a thick rind (exocarp) and fleshy center composed of mesocarp and endocarp. The watermelon fruit, loosely considered a type of melon (although not in the genus Cucumis), has a smooth exterior green and yellow rind and a juicy, sweet, usually red or yellow, but sometimes orange, interior flesh. The plants produce male and female flowers separately on the same plant, and the female flowers have inferior ovaries. The flesh consists of highly developed placental tissue within the fruit.
All parts of the watermelon fruit are used by people, including the fleshy interior, the seeds, and the rind. The fresh may be eaten fresh or used to flavor drinks, and is an excellent source of vitamins C and A, as well as a good source of vitamin B6 and vitamin B1 and various minerals. The seeds are eaten as a snack or added to other dishes, and may be roasted and seasoned. They are a good source of fat and protein. The rind is edible and may be stir-fried, stewed, pickled or even grilled. Beyond these values, the watermelon plant provides aesthetic delight and the fruit an appeal to the senses of taste, sight, and touch. Ecologically, the watermelon flowers provide a source of nectar and pollen for bees.
American humorist Mark Twain, in one of his novels, had a character colorfully refer to the watermelon as “chief of this world’s luxuries, king by the grace of God over all the fruits of the earth. When one has tasted it, he knows what the angels eat. It was not a Southern watermelon that Eve took; we know it because she repented.
Source: New World Encyclopedia