noun
Definition: A type of organism that is made up of a fungus and an alga living together in a close relationship, often found on rocks or tree trunks, and can appear crusty or branching.
Example: The lichen on the old tree trunk was a beautiful shade of green and gray.
transitive verb
Definition: A type of organism that consists of a fungus and an alga or cyanobacterium living together in a symbiotic relationship, often found on rocks, trees, and soil.
Example: The rocks were covered in bright green lichen.