A dictionary is a reference book that contains words and their definitions. It is an alphabetically arranged collection of words, typically including information about their pronunciation, etymology, usage, synonyms, and antonyms. Dictionaries are used to look up the meaning of words, or to find the spelling of a word when you know its definition. They can also be used to learn the correct use of a word in a sentence or phrase. Dictionaries are most commonly printed on paper or in electronic form but they can also be found online. The oldest known dictionary is the Erya which dates back to around 300 BC. Today there are many different kinds of dictionaries such as general dictionaries, specialized dictionaries, and bilingual dictionaries. A general dictionary contains all types of words from different fields such as science and technology, medicine, literature and art. It also includes common phrases and idioms as well as slang terms. Specialized dictionaries contain terms within specific fields such as law or economics. Bilingual dictionaries are useful for people who want to learn a foreign language by providing translations between two languages. Dictionaries can provide definitions for single words or groups of words such as phrasal verbs or collocations which are combinations of two or more words that have a specific meaning when used together. They can also provide example sentences with each definition so that users understand how to correctly apply the word in context. Thesauruses are similar to dictionaries but they contain synonyms rather than definitions for each word listed in them. Thesauruses help users expand their vocabulary by providing alternative options when expressing themselves in writing or speaking without repeating themselves too often with the same words over time.