The general meaning of Thesaurus is a “Treasure” or “Treasury”.

Dr. Peter Mark Roget compiled a Thesaurus of English words based on his system of classification.

A Thesaurus complements a Dictionary.

One looks up a Dictionary when one knows a word and wants to find out the meaning(s).

One looks up a Thesaurus when one has an idea or a concept, and wants to find out the right word.

Over the years, both have evolved and even converged.

One Thesaurus is arranged in Dictionary form for ease of look up.

Advanced Dictionaries include synonyms, antonyms and usage

There are Editors and Word Processors with built-in dictionary and thesaurus.

There is a “Visual Thesaurus” where one can start with a word and follow the links.