lichen

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A composite organism made up of a fungus, usually an ascomycete, that grows symbiotically with an alga or a cyanobacterium and characteristically forms a crustlike or branching growth on rocks or tree trunks.
• noun Medicine Any of various skin diseases characterized by patchy eruptions of small, firm papules.
• transitive verb To cover with lichens.

from The Century Dictionary.

• noun In botany, a plant or vegetable growth of the group Lichenes, ordinarily recognizable by its dry aspect and gray, brown, greenish, or blackish color, and its appearance in crusts, scaly patches, or bush-like forms on trees, rails, rocks, etc.
• noun In pathology, an eruption of papules, of a red or pale color, which do not reach a vesicular or pustular stage. They may be in clusters or scattered, or disseminated over the surface of the skin; and may be attended with itching, as in lichen ruber, or may be quite free from it, as in lichen scrofulosorum.
• To lichenize.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) One of a class of cellular, flowerless plants, (technically called Lichenes), having no distinction of leaf and stem, usually of scaly, expanded, frond-like forms, but sometimes erect or pendulous and variously branched. They derive their nourishment from the air, and generate by means of spores. The species are very widely distributed, and form irregular spots or patches, usually of a greenish or yellowish color, upon rocks, trees, and various bodies, to which they adhere with great tenacity. They are often improperly called rock moss or tree moss.
• noun (Med.) A name given to several varieties of skin disease, esp. to one characterized by the eruption of small, conical or flat, reddish pimples, which, if unchecked, tend to spread and produce great and even fatal exhaustion.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of many symbiotic organisms, being associations of fungi and algae; often found as white or yellow patches on old walls, etc.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks
• noun any thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc.