Any of the deciduous shrubs and trees of the genus Alnus in the birch family (Betulaceae). There are about 30 species, 10 in the United States, widespread in cool north temperate regions, and southward in the Andes of South America. They have a smooth gray bark, elliptical or ovate saw-toothed leaves in three rows, male flowers in long catkins mostly in early spring, and clusters of several dry hard ellipsoid blackish fruits 0.5–1 in. (1.2–2.5 cm) long. The fruits are conelike and present throughout the year.

Alders are common in wet soils, such as stream borders. They often form thickets even beyond treelines and are pioneers on bare areas such as landslides, roadsides, and after fire or logging. As in legumes, the roots bear swellings or nodules containing nitrogen-fixing bacteria which enrich the soil.