A fungus ( ) is a eukaryotic organism that is a member of the kingdom Fungi ( or /ˈfʌŋɡaɪ/). The fungi are a monophyletic group, also called the Eumycota ("true fungi" or eumycetes), that is phylogenetically distinct from the morphologically similar slime molds (myxomycetes) and water molds (oomycetes). The fungi are heterotrophic organisms possessing a chitinous cell wall, with most fungal species growing as multicellular filaments called hyphae forming a mycelium; certain fungal species also grow as single cells. Sexual and asexual reproduction of the fungi is commonly via spores, often produced on specialized structures or in fruiting bodies. Some have lost the ability to form reproductive structures, and propagate solely by vegetative growth. Yeasts, molds, and mushrooms are examples of fungi. The fungi are more closely related to animals than plants, yet the discipline of biology devoted to the study of fungi, known as mycology, is often regarded as a branch of botany.

Occurring worldwide, most fungi are largely invisible to the naked eye, living for the most part in soil, dead matter, and as symbionts of plants, animals, or other fungi. They perform an essential role in ecosystems in decomposing organic matter and are indispensable in nutrient cycling and exchange. Fungi may become noticeable when fruiting, either as mushrooms or molds. They have long been used as a direct source of food, such as mushrooms and truffles and in fermentation of various food products, such as wine, beer, and soy sauce. More recently, fungi are being used as sources for antibiotics used in medicine and various enzymes, such as cellulases, pectinases, and proteases, important for industrial use or as active ingredients of detergents. Many fungi produce bioactive compounds called mycotoxins, such as alkaloids and polyketides that are toxic to animals including humans. Fruiting structures of a few fungal species are used recreationally or in traditional ceremonies as a source of psychotropic compounds. Fungi are significant pathogens of humans and other animals, and losses due to diseases of crops (e.g., rice blast disease) or food spoilage caused by fungi can have a large impact on human food supply and local economies.

Etymology

The English word fungus is directly adopted from the Latin fungus, meaning "mushroom", used in Horace and Pliny. This in turn is derived from the Greek word sphongos/σφογγος ("sponge"), referring to the macroscopic structures and morphology of mushrooms and molds, and this root is also used in other languages (e.g., the German Schwamm ("sponge"), Schimmel ("mold"), or Schwammerl for certain types of mushrooms). The use of the word mycology to denote the scientific study of fungi seems to have originated in 1836 with Miles Joseph Berkeley in his publication The English Flora of Sir James Edward Smith, Vol. 5.Ainsworth, p. 2.