There are many types of dictionaries. Most commonly, dictionaries provide definitions of words, along with other information like: pronunciation, history of the word, or its use in a sentence. Dictionaries may also provided translations of words (such as in a Mandarin-English dictionary) or focus on words used within particular disciplines (like Stedman's Medical Dictionary).
Thesauri typically provide synonyms and antonyms to particular words, helping you find just the right one to use in a sentence.
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Australian Dictionary of Biography by 
Loans in Colonial and Modern Nahuatl by 
A Dictionary of Hallucinations by 
Dictionary of Environmental Health by 
A Dictionary of Literary Devices by