means A composite organism formed by a fungus and an alga (or cyanobacterium) living together, growing as a crusty, leafy, or branching patch on rocks, bark, and bare ground.

from From Greek 'leichēn,' which named these clinging growths and is thought to derive from 'leichein,' meaning 'to lick' — as if the lichen were tonguing the stone it spreads across. The same root sense possibly nods to the way the patch seems to lap at a surface. English borrowed it through Latin 'lichen,' keeping both the spelling and the slightly debated pronunciation (some say 'LY-ken,' some 'LITCH-en').