lichen: Meaning and Definition of

li•chen

1. 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.
2. any of various eruptive skin diseases.

1. to cover with or as if with lichens.