The definition of a lichen is a plant found on rocks or trees made of both a specific fungus and a specific algae that help one another.
An example of a lichen is the colored patch growing on a tree branch.
Lichen on tree bark.
lichen definition by Webster's New World
any of various small plants composed of a particular fungus and a particular alga growing in an intimate symbiotic association and forming a dual plant, commonly adhering in colored patches or spongelike branches to rock, wood, soil, etc.
any of various skin diseases characterized by papules and enlarged skin markings
Origin: Classical Latin ; from Classical Greek leichēn, probably ; from leichein, to lick
A fungus, usually of the class Ascomycetes, that grows symbiotically with algae, resulting in a composite organism that characteristically forms a crustlike or branching growth on rocks or tree trunks.
Pathology Any of various skin diseases characterized by patchy eruptions of small, firm papules.
transitive verbli·chened, li·chen·ing, li·chens
To cover with lichens.
Origin: Latin līchēn, a kind of plant, from Greek leikhēn, from leikhein, to lick; see leigh- in Indo-European roots.
The mutualistic symbiotic association of a fungus with an alga or a cyanobacterium, or both. The fungal component of a lichen absorbs water and nutrients from the surroundings and provides a suitable environment for the alga or cyanobacterium. These live protected among the dense fungal hyphae and produce carbohydrates for the fungus by photosynthesis. Owing to this partnership, lichens can thrive in harsh environments such as mountaintops and polar regions. The more familiar lichens grow slowly as crusty patches, but lichens are found in a variety of forms, such as the tall, plantlike reindeer moss. The association between the different organisms in a lichen is so close that lichens are routinely referred to as a single organism, and scientists classify lichens using the name of the fungal component.