A thesaurus is a dictionary of synonyms (words that have the same or similar meaning). Thesauruses (or thesauri) often include related words that mean almost the same thing. Some thesauruses also include antonyms (words that have the opposite meaning).
For example, by looking up the word "big" in a thesaurus, someone would find a list of other similar words such as "huge" and "massive". If the thesaurus also has a list of antonyms, the user could also find words which mean the opposite, like "small" and "tiny".