A mushroom (or poisonous fungus) is a fleshy, spore bearing fungal fruiting body, usually produced above the ground in soil or food sources.

The standard of the name "mushroom" is the cultivated white button mushroom Agaricus bisporus; therefore, the word "mushroom" is most commonly used for those fungi (Basidiomycota, agaricomycocetes) with stem (stem), cap (villus) and G (cap, monolayer). These G produce tiny spores that help the fungus spread on the ground or on the surface of its occupants.

"Mushroom" describes all kinds of gilded fungi with or without stems, and the term is even more commonly used to describe the succulent fruiting bodies of some ascomycetes and the woody or leather like bodies of some basidiomycetes, depending on the word.

Forms deviating from the standard form usually have more specific names, such as "Boletus", "marbo", "croissant" and "Morchella". The gilded mushrooms themselves are often referred to as "agar" because they are ordered Agaricales with Agaricus blazei or their similarity. By extension, the term "mushroom" can also refer to the whole fungus when cultivated. The mycelium that forms the fruiting body is called the mushroom, or the species itself.