This refers to solvents at elevated pressure and temperature. The exact definition varies but Gerard Demazeau who was a much loved member of the ISHA community defined  solvothermal process as “a chemical reaction in a closed system in the presence of a solvent (aqueous and non aqueous solution) at a temperature higher than that

of the boiling point of such a solvent”. Consequently a solvothermal process involves high

pressures. The selected temperature (sub- or supercritical domains) depends on the required

reactions for obtaining the target-material through the involved process.  

Websters dictionary gives the following definition, having to do with hot water; esp., having to do with the action of hot water in producing minerals and springs or in dissolving, shifting, and otherwise changing the distribution of minerals in the earth's crust. However, ISHA members are interested in using hot water (normally pressurised) for a wide variety of purposes, and not just for geo-applications/studies.

ISHA, by definition covers science and technologies using solvothermal and hydrothermal processes. 

This means that ISHA members work in many many areas including:

nanoparticle formation

formulation science

environmental applications

waste oxidation

geothermal technologies

catalysis

crystallography

advanced metrology

The following websites are worth a looksee 

Note that they have been recommended by members of ISHA, rather than ISHA as an organisation.