A dictionary is a book of alphabetically listed words in a specific language, with definitions, etymologies, pronunciations, and other information; or a book of alphabetically listed words in one language with their equivalents in another, also known as a lexicon. In many languages, words can appear in many different forms, but only the underlined 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 Star Dict, are dictionary software products running on PDAs or computers. There are also many online dictionaries accessible via the Internet. Today, dictionaries of most languages with alphabetic and syllabic writing systems list words in lexicographic order, usually alphabetical or some analogous phonetic system.

In many languages, words are grouped together according to their root word, with the roots being arranged alphabetically. If English dictionaries were arranged like this, the words "import," "export," "support," "report," "porter," "important" and "transportation" would theoretically be listed under the Latin "portare," "to carry." This method has the advantage that all words of a common origin are listed together, but the disadvantage is that one has to know the roots of the word before one can look it up. Some Hebrew, Sanskrit, and Arabic dictionaries work this way. While most of Japanese and Korean dictionaries are arranged according to their phonetic writing (kana syllabic script for the Japanese, and hangul alphabet for the Korean), the main body of modern Chinese dictionaries mostly is ordered according to the Latin alphabet with the pinyin spelling ; but most Chinese dictionaries have an appendix ordering entries accordance to the Chinese logographic writing system , in order to allow readers to find words written in logograms whose pronunciation is not known. Chinese characters may be sorted according to one of many schemes based on the component parts of the characters (radicals, number of strokes, overall shape). Since words and their meanings develop over time, dictionary entries are organized to reflect these changes. Dictionaries may either list meanings in the historical order in which they appeared, or may list meanings in order of popularity and most common use.
Dictionaries also differ in the degree to which they are encyclopedic, providing considerable background information, illustrations, and the like, or linguistic, concentrating on etymology, nuances of meaning, and quotations demonstrating usage. Any dictionary has been designed to fulfill one or more functions. The dictionary functions, or jobs, chosen by the maker(s) of the dictionary provide the basis for all lexicographic decisions, from the selection of entry words, over the choice of information types, to the choice of place for the information (e.g. in an article or in an appendix). There are two main types of functions. The communication-oriented functions comprise text reception (understanding), text production, text revision, and translation. The knowledge-oriented functions deal with situations where the dictionary is used for acquiring specific knowledge about a particular matter, and for acquiring general knowledge about something. The optimal dictionary is one that contains information directly relevant for the needs of the users relating to one or more of these functions. It is important that the information is presented in a way that keeps the lexicographic information costs at a minimum.