A dictionary is a collection of words in one or more specific languages, often listed alphabetically, 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
In linguistics, the lexicon of a language is its vocabulary, including its words and expressions. A lexicon is also a synonym of the word thesaurus. More formally, it is a language's inventory of lexemes. Coined in English 1603, the word "lexicon" derives from the Greek "λεξικόν" , neut...

. According to Nielsen
Sandro Nielsen
Sandro Nielsen is a Danish metalexicographer associated with Centre for Lexicography at the Aarhus School of Business, Denmark, from where he received his PhD in 1992. Nielsen has contributed to lexicography as a theoretical and practical lexicographer with particular reference to bilingual...

 (2008) a dictionary may be regarded as a lexicographical
Lexicography is divided into two related disciplines:*Practical lexicography is the art or craft of compiling, writing and editing dictionaries....

 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 fulfill the functions of the dictionary.

A broad distinction is made between general and specialized dictionaries.