Fraxinus ( /ˈfræksɨnəs/) is a genus of flowering plants in the olive and lilac family, Oleaceae. It contains 45-65 species of usually medium to large trees, mostly deciduous though a few subtropical species are evergreen. The tree's common English name, ash, goes back to the Old English æsc, while the generic name originated in Latin. Both words also meant "spear" in their respective languages. The leaves are opposite (rarely in whorls of three), and mostly pinnately compound, simple in a few species. The seeds, popularly known as keys or helicopter seeds, are a type of fruit known as a samara. Rowans or Mountain Ashes are unrelated to true ashes and belong to the Genus Sorbus though the leaves and buds are superficially similar.

Ash is used as a food plant by the larvae of some Lepidoptera species (butterflies and moths) -- see list of Lepidoptera that feed on ashes.

The emerald ash borer (Agrilus planipennis) is a wood-boring beetle accidentally introduced to North America from eastern Asia via solid wood packing material in the late 1980's to early 1990's. It has killed tens of millions of trees in 15 states in the United States and adjacent Ontario. It threatens some 7 billion ash trees in North America. Research is being conducted to determine if three native asian wasps, who are natural predators of EAB could be used as a biological control for the management of EAB populations in the United States. The public is being cautioned not to transport unfinished wood products, such as firewood, to slow the spread of this insect pest.

Ash may refer to:

The solid remains of fires, such as:

A tree is a perennial woody plant. It most often has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to 6 m; some authors set a minimum of 10 cm trunk diameter (30 cm girth). Woody plants that do not meet these definitions by having multiple stems and/or small size are usually called shrubs, although many trees such as Mallee do not meet such definitions. Compared with most other plants, trees are long-lived, some reaching several thousand years old and growing to up to 115 m (379 ft) high.

Trees are an important component of the natural landscape because of their prevention of erosion and the provision of a weather-sheltered ecosystem in and under their foliage. They also play an important role in producing oxygen and reducing carbon dioxide in the atmosphere, as well as moderating ground temperatures. They are also elements in landscaping and agriculture, both for their aesthetic appeal and their orchard crops (such as apples). Wood from trees is a building material, as well as a primary energy source in many developing countries. Trees also play a role in many of the world's mythologies (see trees in mythology).