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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.]