Ash, the common name for a genus of hardy ornamental and commercially useful trees. All members of this genus are commonly called ashes. There are about 65 species, which are found in temperate regions of the Northern Hemisphere; a few species are also found in tropical regions. There are more than 15 species in the United States. The white ash provides one of the leading commercial hardwoods in the United States.

Other species in the United States include the red, green, blue, black, and Oregon ashes—all sources of hardwood. Ash wood, which is strong and flexible, is light reddish-brown. This wood is used for tool handles, oars, baseball bats, clothespins, toys, and many other items. It is also used for furniture and interior trim of buildings. Many ashes are grown as ornamentals.

Ash trees range in height from about 15 to 120 feet (4.6 to 37 m), depending on the species. They have saw-toothed leaves that typically turn purple or yellow in autumn. Ash trees bear small white, yellow, green, or purple flowers, and winged seeds called keys, or samaras.

The name ash is also applied to various trees that are not of the ash genus, such as the mountain ash, the prickly ash, and the bitter ash, a source of quassia.

Ashes make up the genus Fraxinus of the olive family, Oleaceae. The white ash is F. americana. Red ash is F. pennsylvanica. Black ash is F. nigra.