A dictionary is a book (physical or digital) in which the words used by a language are defined so that users can have a standard reference for a word's meaning. Older dictionaries primarily focused on uncommon and lesser used words since everyone knew the common ones, but modern dictionaries include common and often-used words as well. For the English language, the major dictionaries are Roget's, Merriam-Webster, and the huge big daddy of English dictionaries, Oxford's English Dictionary, whose Second Edition reached 20 volumes and 3 supplements and whose Third Edition is likely to end up so massive that it will not be published in physical form.

More specific types of dictionaries also exist, such as thesauruses, rhyming dictionaries, and specialized dictionaries that cover particular subject fields. Also, humorous dictionaries such as Douglas Adams' The Meaning of Liff or Ambrose Bierce's The Devil's Dictionary also exist.

For an online dictionary you may find useful, try Wiktionary

More information:

https://en.wikipedia.org/wiki/Dictionary