(entomology, botany) A chamber produced by a plant in which insects, mites, or fungi live. It typically takes the form of a hollow under a leaf, or a system of tunnels in a thorn or stem. Ideally a domatium is a mutualistic adaptation, not to be confused with simple damage by a borer or gall-forming pest, though commonly there is no sharp distinction between domatia of value to the plant and galls caused by harmful aphids and mites for example.