lichen

noun

• 1a simple slow-growing plant which typically forms a low crust-like, leaf-like, or branching growth on rocks, walls, and trees: the cedars are festooned with lichen [count noun]:rocks covered with gold lichens

Lichens are composite plants consisting of a fungus that contains photosynthetic algal cells. Their classification is based upon that of the fungal partner, which in most cases belongs to the subdivision Ascomycotina, and the algal partners are either green algae or cyanobacteria

• 2 [mass noun, usually with modifier] a skin disease in which small, hard round lesions occur close together: patients with oral lichen planus