<p><strong>Noun</strong></p><ul><li>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.</li><li>Pathology: Any of various eruptive skin diseases.</li></ul><p><strong>Verb</strong></p><ul><li>(Used with object) To cover with or as if with lichens.</li></ul>