Wiktionary
cystidium
n. (context mycology English) A relatively large cell found on the hymenium of a basidiomycete
Wikipedia
Cystidium
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.
Usage examples of "cystidium".
In the family called Hymenomycetes there are mixed with these, and closely packed together, one-celled sterile structures named cystidia.