Definition of thesaurus
WHAT IS A THESAURUS?
A reference book containing a classified list of synonyms which also provides an analysis -a definition- of the meaning or meanings of each entry word in the book. In lexicography, they are sometimes referred to as ‘dictionaries of usage’, i.e. specifically designed to facilitate or improve the productive use of the language.
It should be noted that Thesauruses are monolingual reference books.