(Science: botany) 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 European shrub (rhamnus frangula); alder buckthorn. An American species of holly (ilex verticillata), bearing red berries.