NOUN (2)

1. any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks;

2. any thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc.;

The Collaborative International Dictionary of English v.0.48:

Lichen \Li"chen\ (l[imac]"k[e^]n; 277), n. [L., fr. Gr.
   leichh`n.]
   1. (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.
      [1913 Webster]

   Note: 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 alg[ae]. 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 Hyph[ae], and Gonidia.
         [1913 Webster]

   2. (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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lichen
    n 1: any of several eruptive skin diseases characterized by hard
         thick lesions grouped together and resembling lichens
         growing on rocks
    2: any thallophytic plant of the division Lichenes; occur as
       crusty patches or bushy growths on tree trunks or rocks or
       bare ground etc.

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "lichen":
   acne, acne vulgaris, algae, autophyte, bean, bracken, brown algae,
   climber, conferva, confervoid, creeper, dermamycosis, dermatitis,
   dermatosis, diatom, eczema, elephantiasis, epithelioma, erysipelas,
   erythema, exanthem, fern, fruits and vegetables, fucus, fungus,
   grapevine, green algae, gulfweed, heat rash, herb, herpes,
   herpes simplex, herpes zoster, heterophyte, hives, impetigo, itch,
   ivy, jungle rot, kelp, legume, lentil, leprosy, liana,
   lichen primus, liverwort, lupus, lupus vulgaris, miliaria, mold,
   moss, mushroom, parasite, parasitic plant, pea, pemphigus,
   perthophyte, phytoplankton, planktonic algae, plant families,
   prickly heat, pruigo, pruritus, psora, puffball, pulse, red algae,
   ringworm, rockweed, rust, saprophyte, sargasso, sargassum, scabies,
   sea lentil, sea moss, sea wrack, seaweed, shingles, skin cancer,
   smut, succulent, tetter, toadstool, vetch, vine, wort, wrack