A thesaurus is a reference tool that provides synonyms and antonyms for words. It helps users find alternative words with similar meanings, enhancing vocabulary and improving writing. Thesauri can be found in print or online, making them accessible for various writing needs. Using a thesaurus can aid in avoiding repetition and finding the right word for a specific context. For example, instead of using the word happy repeatedly, one might choose joyful, content, or cheerful to add variety. This tool is valuable for students, writers, and anyone looking to express themselves more effectively.