any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocksany thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc.
英汉双解大词典
lichen/ˈlaɪkən/star1TEM4(lichens)
1.
N-MASS（集合名词） Lichen is a group of tiny plants that looks like moss and grows on the surface of things such as rocks, trees, and walls. 地衣