lichen Definition

any of numerous complex plantlike organisms made up of an alga or a cyanobacterium and a fungus growing in symbiotic association on a solid surface (such as on a rock or the bark of trees)

any of several skin diseases characterized by the eruptions of flat papules

a gray, erect, tufted, and much-branched lichen (Cladonia rangiferina) that forms extensive patches in arctic and north-temperate regions, constitutes a large part of the food of caribou, and is sometimes eaten by humans —called also reindeer lichen