Real Dictionary

lichen
variable noun: Lichen is a group of tiny plants that looks like moss and grows on the surface of things such as rocks, trees, and walls.

lichen in American English
any of various small plants composed of a particular fungus and a particular alga growing in an intimate symbiotic association and forming a dual plant, commonly adhering in colored patches or spongelike branches to rock, wood, soil, etc.
to cover with lichens
noun: any complex organism of the group Lichenes, composed of a fungus in symbiotic union with an alga and having a greenish, gray, yellow, brown, or blackish thallus that grows in leaflike, crustlike, or branching forms on rocks, trees, etc
transitive verb: to cover with or as if with lichens

lichen in British English
noun: an organism that is formed by the symbiotic association of a fungus and an alga or cyanobacterium and occurs as crusty patches or bushy growths on tree trunks, bare ground, etc. Lichens are now classified as a phylum of fungi (Mycophycophyta)