Symbiosis: A close and long-lasting association between two or more organisms of different species. The three types are mutualism, commensualism, and parasitism.

Fungal partner: It provides protection to the photosynthetic partner by producing a thallus. It cannot live without the energy provided by the photosynthetic partner.

Photosynthetic partner: It has the ability to produce carbohydrates by photosynthesis. In a lichen, this energy provides for itself and the fungal partner, and is either an algae or cyanobacteria.

Mutualism: A symbiotic relationship in which each organism benefits from the association.

Controlled parasitism: A parasitism is a symbiotic relationship in which a certain species benefits at the expense of another. A controlled parasitism is when the organisms in association are in equilibrium and there appear to be no or minimal damage or disruption to the host organism.

Thallus: The vegetative body of a lichen. It is produced by the fungal partner and houses the photosynthetic partner.

Cortex: In lichens, it is the outer "skin" that forms a protective cover. It is made up of fungal hyphae and is usually smooth.

Isidia: In lichens, asexual reproductive structures. They are grown out of the thallus, are covered with cortex, and include the photosynthetic partner.

Soredia: In lichens, asexual reproductive structures that contain both the photosynthetic partner and the fungal partner. Their appearance ranges from powder to granules, and they are not covered by cortex.

Asci: They are involved in sexual reproduction. In ascomycetes including lichens, it is a sac-like structure where sexually produced spores are formed.

Ascomata (also called ascocarp): The sexual reproductive bodies of an ascomycete. They are the location where the asci are formed.

Apothecium: In lichens, it is a type of ascomata (reproductive body). It is usually shaped like an open disc.

Secondary Compounds: Products of lichens that would otherwise have not been produced by either partner. They can be used for identification purposes as well as having many many practical purposes.