lichen

Concise Oxford English Dictionary © 2008 Oxford University Press:
lichen /ˈlʌɪk(ə)n, ˈlɪtʃ(ə)n/

▶noun

• 1 a simple composite plant consisting of a fungus in association with an alga, typically growing on rocks, walls, and trees.

• 2 a skin disease in which small, round, hard lesions occur close together.

– derivatives
lichened adjective,
lichenology noun,
lichenous adjective.

– origin C17: via L. from Gk leikhēn.