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1. 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.
2. Medicine Any of various skin diseases characterized by patchy eruptions of small, firm papules.
tr.v. li·chened, li·chen·ing, li·chens
To cover with lichens.

[Latin līchēn, a kind of plant, from Greek leikhēn, from leikhein, to lick; see leigh- in Indo-European roots.]

li′chen·ous adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
References in classic literature ?
An emanation, a particular spirit, belonged not to the moving leaves or water only, but to the distant peak arising suddenly, by some change of perspective, above the nearer horizon of the hills, to the passing space of light across the plain, to the lichened Druidic stone even, for a certain weird fellowship in it with the moods of men.
Against the peaceful landscape, the pale, decaying tints of the copses, the blue air of the horizon and the lichened stileboards, these staring vermilion words shone forth.