Množina: lichens

ETYM Latin, from Greek leichen.
(Homonym: liken).
Any thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc.
Any organism of the group Lichenes, which consists of a specific fungus and a specific alga existing in a mutually beneficial relationship. Found as colored patches or spongelike masses adhering to trees, rocks, and other substrates, lichens flourish under adverse conditions.
Some lichens have food value, for example, reindeer moss and Iceland moss; others give dyes, such as litmus, or are used in medicine. They are sensitive to atmospheric pollution (see indicator species).
Lowly gray-green flowerless plant encrusting rocks, trees, etc.; encrusting skin disease.

Singular of lichens is lichen.