On this Wikipedia the language links are at the top of the mushroom sauce across from the article title. For use in food, see Edible mushroom. A mushroom or toadstool is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground, on soil, or on its food source. Toadstool generally denotes one poisonous to humans.

Amanita muscaria, the most easily recognised “toadstool”, is frequently depicted in fairy stories and on greeting cards. It is often associated with gnomes. The terms “mushroom” and “toadstool” go back centuries and were never precisely defined, nor was there consensus on application. During the 15th and 16th centuries, the terms mushrom, mushrum, muscheron, mousheroms, mussheron, or musserouns were used. Delineation between edible and poisonous fungi is not clear-cut, so a “mushroom” may be edible, poisonous, or unpalatable.

Identifying what is and is not a mushroom requires a basic understanding of their macroscopic structure. Their spores, called basidiospores, are produced on the gills and fall in a fine rain of powder from under the caps as a result. While modern identification of mushrooms is quickly becoming molecular, the standard methods for identification are still used by most and have developed into a fine art harking back to medieval times and the Victorian era, combined with microscopic examination. In general, identification to genus can often be accomplished in the field using a local field guide. Identification to species, however, requires more effort. A mushroom develops from a button stage into a mature structure, and only the latter can provide certain characteristics needed for the identification of the species.

Typical mushrooms are the fruit bodies of members of the order Agaricales, whose type genus is Agaricus and type species is the field mushroom, Agaricus campestris. Other mushrooms are not gilled, so the term “mushroom” is loosely used, and giving a full account of their classifications is difficult. A mushroom develops from a nodule, or pinhead, less than two millimeters in diameter, called a primordium, which is typically found on or near the surface of the substrate. It is formed within the mycelium, the mass of threadlike hyphae that make up the fungus. In other mushrooms, a stalk may be absent, as in the polypores that form shelf-like brackets. Puffballs lack a stalk, but may have a supporting base. The way the gills attach to the top of the stalk is an important feature of mushroom morphology.

Mushrooms in the genera Agaricus, Amanita, Lepiota and Pluteus, among others, have free gills that do not extend to the top of the stalk. A hymenium is a layer of microscopic spore-bearing cells that covers the surface of gills. In the nongilled mushrooms, the hymenium lines the inner surfaces of the tubes of boletes and polypores, or covers the teeth of spine fungi and the branches of corals. In the basidiomycetes, usually four spores develop on the tips of thin projections called sterigmata, which extend from club-shaped cells called a basidia. The most important microscopic feature for identification of mushrooms is the spores.