An oak is a tree or shrub in the genus Quercus (/ˈkwɜːrkəs/; Latin "oak tree") of the beech family, Fagaceae. There are approximately 600 extant species of oaks. The common name "oak" may also appear in the names of species in related genera, notably Lithocarpus. The genus is native to the Northern Hemisphere, and includes deciduous and evergreen species extending from cool temperate to tropical latitudes in the Americas, Asia, Europe, and North Africa. North America contains the largest number of oak species, with approximately 90 occurring in the United States, while Mexico has 160 species of which 109 are endemic. The second greatest center of oak diversity is China, which contains approximately 100 species. Oaks have spirally arranged leaves, with lobate margins in many species; some have serrated leaves or entire leaf with smooth margins. Many deciduous species are marcescent, not dropping dead leaves until spring. In spring, a single oak tree produces both male flowers (in the form of catkins) and small coon flowers. The fruit is a nut called an acorn, borne in a cup-like structure known as a cupule; each acorn contains one seed (rarely two or three) and takes 6–18 months to mature, depending on their species. The live oaks are distinguished for being evergreen, but are not actually a distinct group and instead are dispersed across the genus.

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