A geothermal resource can be defined as a reservoir inside the Earth from which heat can be extracted economically Geothermal resources are classified on the basis of different aspects, such as heat source, heat transfer, reservoir tem perature, physical state, commercial utilization and geological settings. Unfortunately most of the current classifications that are used for geothermal systems are not complete. So, a combinational terminology of geological and tempera ture-based classifications would be more complete. This terminology can explain all geological situations, temperature and physical state of geothermal reservoir altogether. According to geological settings, in combinational terminology (from left to right), the class of geothermal resource’s name would be placed at first, then the physical state of reservoir (Liquid-dominated or Two-phase or Vapor-dominated) would be written and finally the class of the geothermal reser voir which is related to its temperature, is written.