This dictionary is the first attempt to standardize hydrogeological terms and concepts in order to improve cooperation between specialists in this field and to reduce the possibility of misinterpretations when reading and using hydrogeological literature by specialists from different fields. The dictionary includes 1943 terms and concepts in six languages, with semantic explanations given only in Bulgarian. At the end, alphabetical indexes of the terms in the six languages ​​are given with the corresponding number under which they can be found in Bulgarian. Thus, for each term, not only its semantic meaning can be found, but also its corresponding equivalents in other languages.