Lichen (noun)

1. A composite organism made up of a fungus and an alga or cyanobacterium.
2. A type of plant that consists of a fungus and an alga living together in a symbiotic relationship.

Origin:

Old english lycen, from latin lichen, from greek leichen "lichen", of uncertain origin.

Examples:

1. The lichen on the rock was a beautiful green color.
2. Lichens are found in many different environments, including on rocks, trees and soil.
3. Lichens are important indicators of air quality.
4. The lichen is slow-growing and can live for centuries.
5. Lichens are used in traditional medicine and as a source of food and dye.