An oak is a tree or shrub in the genus Quercus ( /ˈkwɜrkəs/; Latin "oak tree"), of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus. The genus is native to the Northern Hemisphere, and includesdeciduous and evergreen species extending from cool temperate to tropical latitudes in Asia and the Americas.
Oaks have spirally arranged leaves, with lobed margins in many species; some have serrated leaves or entire leaves with smooth margins. Many deciduous species are marcescent, not dropping dead leaves until spring. The flowers are catkins, produced in spring. 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 species. The live oaks are distinguished for being evergreen, but are not actually a distinct group and instead are dispersed across the genus.