A basidium (pl., basidia) is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. The presence of basidia is one of the main characteristic features of the Basidiomycota. A basidium usually bears four sexual spores called basidiospores; occasionally the number may be two or even eight. In a typical basidium, each basidiospore is borne at the tip of a narrow prong or horn called a sterigma (pl. sterigmata), and is forcibly discharged upon maturity.
The word basidium literally means little pedestal, from the way in which the basidium supports the spores. However, some biologists suggest that the structure more closely resembles a club. An immature basidium is known as a basidiole.