The following is a glossary of scientific terminology, terms used in the description of lichens, composite organisms that arise from algae or cyanobacteria living among Hypha, filaments of multiple fungus species in a Mutualism (biology), mutualistic relationship.

Until the end of the 18th century, only a couple of lichen-specific terms had been proposed. Johann Jacob Dillenius, Johann Dillenius introduced ' in 1742 to describe the cup-shaped structures associated with genus ''Cladonia'', while in 1794 Michel Adanson used ' for the furrowed fruitbodies of the genus ''Graphis (lichen), Graphis''. Erik Acharius, the "father of lichenology", conceived many new terms to describe lichen structures in several of his seminal publications in the early 1800s. Examples include , , , , , , and . In 1825, Karl Friedrich Wilhelm Wallroth, Friedrich Wallroth published the first of his multi-volume work ''Naturgeschichte der Flechten'' ("Natural History of Lichens"), in which he proposed an alternative terminology based largely on Root (linguistics), roots from the Greek language. His work, presented as an alternative to that of Acharius (of whom he was critical) was not well received, and the only terms he proposed to gain widespread acceptance were and , and , and , the last of which remained in use until the 1960s. Until about 1850, there were 21 terms for features of the lichen thallus that remain in use today. The increasing availability of the optical microscope as an aid to identifying and characterizing lichens led to the creation of new terms to describe structures that were previously too small to be visualized. Contributions were made by Julius von Flotow (e.g. ), Edmond Tulasne (e.g ), and William Nylander (botanist), William Nylander (e.g. , ). Gustav Wilhelm Körber, an early proponent of using spore structure as a in lichen taxonomy, introduced , , and "polari-dyblastae", later anglicized to "polari-bilocular" and then shortened to . In the next five decades that followed, many other additions were made to the repertoire of lichen terms, subsequent to the increased understanding of lichen anatomy and physiology made possible by microscopy. For whatever reasons, there were not any new terms (still currently used) introduced from the period 1906 to 1945, when Gustaf Einar Du Rietz proposed replacing and with and ; all four terms remain in use. In some cases, older terminology became obsolete as better understanding of the nature of the fungal–algal relationship led to changes in their terminology. For example, after Gunnar Degelius objected to the use of for the algal partner, George Scott proposed the use of and for lichen components, recommendations that were generally accepted by lichenologists. Terms defining features of lichens are used to describe aspects and relationships unique to lichens as composite organisms. These include terms that describe the two major components of lichens (' and '); specialized structures in lichen physiology (e.g., podetium, , Medulla (lichenology), medulla, the ); descriptors of types of lichens (e.g., , , ); two- and three-dimensional shapes used to describe spores and other lichen structures (e.g., , , ); terms of position (e.g. , ) and shape (e.g. , ); prefixes and suffixes commonly used to form lichen terms; terminology used in methods for the chemical identification of lichens (e.g. various spot test (lichen), spot tests); and "everyday" words that have a specialized meaning in lichenology (e.g., , , ). The list also includes a few historical terms that have been supplanted or are now considered obsolete (e.g., , ). Although no longer part of the modern lichen lexicon, knowledge of these terms helps in understanding older literature.