lichen

[lahy-kuhn]
noun
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.
Pathology. any of various eruptive skin diseases.
verb (used with object)
3.
to cover with or as if with lichens.

Origin:
1595–1605; < Latin līchēn < Greek leichḗn

lichenization, noun
lichenlike, adjective
unlichened, adjective

lichen, liken.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
lichen (ˈlaɪkən, ˈlɪtʃən)
 
n
1.  an organism that is formed by the symbiotic association of a fungus and an alga or cyanobacterium and occurs as crusty patches or bushy growths on tree trunks, bare ground, etc. Lichens are now classified as a phylum of fungi (Mycophycophyta)
2.  pathol any of various eruptive disorders of the skin
 
[C17: via Latin from Greek leikhēn, from leikhein to lick]
 
'lichened
 
adj
 
'lichen-like
 
adj
 
'lichenoid
 
adj
 
'lichenous
 
adj
 
'lichenose
 
adj

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

lichen
c.1600, from L. lichen, from Gk. leichen, originally "what eats around itself," probably from leichein "to lick" (see lick). Originally used of liverwort; the modern sense first recorded 1715.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

lichen li·chen (lī'kən)
n.
Any of various skin diseases characterized by patchy eruptions of small, firm papules.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
lichen   (lī'kən)  Pronunciation Key 
The mutualistic symbiotic association of a fungus with an alga or a cyanobacterium, or both. The fungal component of a lichen absorbs water and nutrients from the surroundings and provides a suitable environment for the alga or cyanobacterium. These live protected among the dense fungal hyphae and produce carbohydrates for the fungus by photosynthesis. Owing to this partnership, lichens can thrive in harsh environments such as mountaintops and polar regions. The more familiar lichens grow slowly as crusty patches, but lichens are found in a variety of forms, such as the tall, plantlike reindeer moss. The association between the different organisms in a lichen is so close that lichens are routinely referred to as a single organism, and scientists classify lichens using the name of the fungal component.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Forest floors are covered with ferns and mosses and the trees themselves drip
  with lichen.
And the bark beneath the white lichen that always covers it is reddish too.
The trees are coming into their winter bareness, the only green is the lichen
  on their branches.
After milking, they release them to graze on lichen.
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