Definition of: lichen

(lī′kən) noun
1. A flowerless plant (class or group Lichenes) commonly growing flat upon a surface, as of a rock, and composed of loose cellular tissue, a slender white–celled epiphytic ascomycetous or (rarely) basidiomycetous fungus, and a number of globular greenish or bluish algal cells upon which the fungal cells prey.
2. Pathol. Any of several papular skin diseases.
—v.t. To cover with lichens. [<L <Gk. leichēn, prob. <leichein lick]
—li′chen·a′ceous (-ā′shəs), li′chen·ous, li′chen·ose (-ōs) adjective