A thesaurus (plural thesauri or thesauruses), sometimes called a synonym dictionary or dictionary of synonyms, is a reference work which arranges words by their meanings into lists of synonyms and antonyms. Most thesauri do not include definitions, but many dictionaries include listings of synonyms. The word "thesaurus" comes from Latin thēsaurus, which in turn comes from Greek θησαυρός (thēsauros) 'treasure, treasury, storehouse'. The word thēsauros is of uncertain etymology.