Of an environment: characterized by moderate temperatures; (of an organism) inhabiting such an environment.
Of, relating to, or designating mineral and ore deposits formed by hydrothermal action at intermediate temperature and pressure.
Of the speed of an object in a plasma: less than that of the ambient electrons but higher than that of the ions.
Early 20th century; earliest use found in Bulletin of the Torrey Botanical Club. From meso- + thermal.