1.  lichen(n = noun.state) - any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks;
is a kind of disease of the skin, skin disease, skin disorder
has particulars: lichen planus, lichen ruber planus

2.  lichen(n = noun.plant) - any thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc.;
is a kind of fungus
is a member of division lichenes, lichenes
has particulars: ascolichen, basidiolichen, lecanora, roccella, roccella tinctoria, beard lichen, beard moss, usnea barbata, horsehair lichen, horsetail lichen, arctic moss, cladonia rangiferina, reindeer lichen, reindeer moss, crotal, crottal, crottle, cetraria islandica, iceland lichen, iceland moss

1.  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. [1913 Webster]
" A favorite modern theory of lichens (called after its inventor the Schwendener hypothesis), is that they are not autonomous plants, but that they consist of ascigerous fungi, parasitic on algæ. Each lichen is composed of white filaments and green, or greenish, rounded cells, and it is argued that the two are of different nature, the one living at the expense of the other. See Hyphæ, and Gonidia." [1913 Webster]

2.  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. [1913 Webster]

lichen, n.
1 any plant organism of the group Lichenes, composed of a fungus and an alga in symbiotic association, usu. of green, grey, or yellow tint and growing on and colouring rocks, tree-trunks, roofs, walls, etc.
2 any of several types of skin disease in which small round hard lesions occur close together.

Derivative:
lichened adj. (in sense 1). lichenology n. (in sense 1). lichenous adj. (in sense 2).

Etymology:
L f. Gk leikhen

N  vegetable, vegetable kingdom, flora, verdure, plant, tree, shrub, bush, creeper, herb, herbage, grass, annual, perennial, biennial, triennial, exotic, timber, forest, wood, woodlands, timberland, hurst, frith, holt, weald, park, chase, greenwood, brake, grove, copse, coppice, bocage, tope, clump of trees, thicket, spinet, spinney, underwood, brushwood, scrub, boscage, bosk, ceja, chaparal, motte, arboretum, bush, jungle, prairie, heath, heather, fern, bracken, furze, gorse, whin, grass, turf, pasture, pasturage, turbary, sedge, rush, weed, fungus, mushroom, toadstool, lichen, moss, conferva, mold, growth, alfalfa, alfilaria, banyan, blow, blowth, floret, petiole, pin grass, timothy, yam, yew, zinnia, foliage, branch, bough, ramage, stem, tigella, spray, leaf, flower, blossom, bine, flowering plant, timber tree, fruit tree, pulse, legume, vegetable, vegetal, vegetive, vegitous, herbaceous, herbal, botanic, sylvan, silvan, arborary, arboreous, arborescent, arborical, woody, grassy, verdant, verdurous, floral, mossy, lignous, ligneous, wooden, leguminous, vosky, cespitose, turf-like, turfy, endogenous, exogenous, green-robed senators of mighty woods, this is the forest primeval.

beard lichen, horsehair lichen, horsetail lichen, iceland lichen, lichen planus, lichen ruber planus, manna lichen, reindeer lichen