A tree, usually growing in moist land, and belonging to the genus Alnus. The wood is used by turners, etc.; the bark by dyers and tanners. In the U. S. the species of alder are usually shrubs or small trees. Black alder. (a) A European shrub (Rhamnus frangula); Alder buckthorn. (b) An American species of holly (Ilex verticillata), bearing red berries.