A dictionary is a book or collection of words in a specific language, often listed alphabetically, with definitions, etymologies, 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 product that is characterised by three significant features:   it has been prepared for one or more functions;   it contains data that have been selected for the purpose of fulfilling those functions; and   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.

In many languages, words can appear in many different forms, but only the undeclined or unconjugated 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 or computers. There are also many online dictionaries accessible via the Internet.