A dictionary is a collection of words, often listed alphabetically, along with their definitions, pronunciations and etymologies, or origins. It is commonly used as a reference tool to find the meanings of unfamiliar words. A dictionary may also include example sentences to help explain the meaning of a word. Dictionaries are typically organized alphabetically by headwords or main entries, which are the words being defined. The definition for each headword is typically provided in a single sentence or phrase that describes its meaning in the context of language usage. The definition may also include synonyms and antonyms, related terms and phrases as well as variant spellings and collocations. Etymologies are sometimes included to provide information on the origin of a word or phrase, including its historical development over time in different cultures and languages. Pronunciations can also be provided in dictionaries to help readers correctly pronounce unfamiliar words. In addition to providing definitions for individual words, some dictionaries may also offer detailed descriptions of idioms and expressions as well as specialized terminology related to specific fields such as medicine or law. A dictionary may also contain information about grammar rules and usage conventions associated with particular languages or dialects. For learners of foreign languages, bilingual dictionaries can be very useful when trying to understand new vocabulary from another language. These types of dictionaries typically provide translations between two languages for each entry word along with additional information about its pronunciation and usage in both languages. Overall, a dictionary can be an invaluable tool for students, language learners or anyone who needs assistance with understanding new words or finding the right words for writing projects and conversations.