A thesaurus is a reference book or digital tool that lists words grouped together according to similarity of meaning. It helps users find synonyms, which are words that have similar meanings, and antonyms, which are words with opposite meanings. This can enhance writing by providing more varied vocabulary. Thesauruses are useful for writers, students, and anyone looking to improve their language skills. They can be found in print or online, with many digital versions offering additional features like example sentences and usage tips. Popular examples include the Roget's Thesaurus and various online platforms like Thesaurus.com.