A cystidium (plural cystidia) is a relatively large cell found on the hymenium of a basidiomycete (for example, on the surface of a mushroom gill), often between clusters of basidia. Since cystidia have highly varied and distinct shapes that are often unique to a particular species or genus, they are a useful micromorphological characteristic in the identification of basidiomycetes. In general, the adaptive significance of cystidia is not well understood.