dictionary, also referred to as a lexicon, wordbook, or vocabulary, is a collection of words in one or more specific languages, often listed alphabetically (Alphabetical order), with usage information, definitions, etymologies, phonetics, pronunciations, and other information; or a book of words in one language with their equivalents in another, also known as a lexicon. According to Nielsen 2008 a dictionary may be regarded as a lexicographical (Lexicography) product that is characterised by three significant features: (1) it has been prepared for one or more functions; (2) it contains data that have been selected for the purpose of fulfilling those functions; and (3) its lexicographic structures link and establish relationships between the data so that they can meet the needs of users and fulfil the functions of the dictionary.

Further, each word may have multiple meanings. A dictionary typically includes each separate meaning in the order of most common usage.

In many languages, words can appear in many different forms, but only the undeclined or unconjugated (lemma (linguistics)) form appears as the headword in most dictionaries. Dictionaries are most commonly found in the form of a book, but some newer dictionaries, like StarDict and the New Oxford American Dictionary are dictionary software running on PDAs (Personal Digital Assistant) or computers. There are also many online dictionaries accessible via the Internet. .