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A pumpkin is a cultivar of a squash plant, most commonly of Cucurbita pepo, that is round, with smooth, slightly ribbed skin, and deep yellow to orange coloration.

The thick shell contains the seeds and pulp. Some exceptionally large cultivars of squash with similar appearance have also been derived from Cucurbita maxima. Specific cultivars of winter squash derived from other species, including C. argyrosperma, and C. moschata, are also sometimes called "pumpkin".
Source: Wikipedia