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. lichened adj. (in sense 1). lichenology n. (in sense 1). lichenous adj. (in sense 2).
[ L f. Gk leikhen ]