The acorn, or oaknut, is the nut of the oaks and their close relatives (genera Quercus and Lithocarpus, in the family Fagaceae). It usually contains one seed (occasionally
two seeds), enclosed in a tough, leathery shell, and borne in a cup-shaped cupule. Acorns are 1–6 cm (1⁄2–2+1⁄2 in) long and 0.8–4 cm (3⁄8–1+5⁄8 in) on the fat side. Acorns take between 5 and 24 months (depending on the species) to mature; see the list of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.

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