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.
Derivatives:
lichened adj. (in sense 1). lichenology n. (in sense 1). lichenous adj. (in sense 2).
Etymology: L f. Gk leikhen