Dictionaries are reference books containing an alphabetical list of words and their definitions and pronunciations. They are often used to look up the meanings of unfamiliar words or to find the correct spelling of a word. They may also include information about a word’s origin, usage, and other related words. The best dictionaries in English depend on the user’s needs. For general purpose use, the Oxford English Dictionary is considered one of the most comprehensive and authoritative dictionaries available. It contains more than 600,000 words and definitions as well as etymologies, quotations, and other reference material. The Merriam-Webster Dictionary is another popular reference for general use that contains more than 225,000 definitions. For learners of English as a second language, the Longman Dictionary of Contemporary English is a popular choice due to its extensive coverage of idiomatic expressions and usage notes to help users understand how words are used in context. The Cambridge Advanced Learner’s Dictionary is another good option for ESL learners with helpful illustrations and examples to aid understanding. Finally, specialized dictionaries such as medical or legal dictionaries can be invaluable resources for professionals in those fields.