A dictionary is a collection of words in one or more specific languages, often listed alphabetically (or by radical and stroke for ideographic languages), 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 product that is characterized 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. Specialized dictionaries do not contain information about words that are used in language for general purposes—words used by ordinary people in everyday situations. Lexical items that describe concepts in specific fields are usually called terms instead of words, although there is no consensus whether lexicology and terminology are two different fields of study.
In theory, general dictionaries are supposed to be semasiological, mapping word to definition, while specialized dictionaries are supposed to be onomasiological, first identifying concepts and then establishing the terms used to designate them. In practice, the two approaches are used for both types. There are other types of dictionaries that don't fit neatly in the above distinction, for instance bilingual (translation) dictionaries, dictionaries of synonyms (thesauri), or rhyming dictionaries. The word dictionary (unqualified) is usually understood to refer to a monolingual general-purpose dictionary.
A different dimension on which dictionaries (usually just general-purpose ones) are sometimes distinguished is whether they are prescriptive or descriptive, the latter being in theory largely based on linguistic corpus studies—this is the case of most modern dictionaries. However, this distinction cannot be upheld in the strictest sense. The choice of headwords is considered itself of prescriptive nature; for instance, dictionaries avoid having too many taboo words in that position. Stylistic indications (e.g. ‘informal’ or ‘vulgar’) present in many modern dictionaries is considered less than objectively descriptive as well.
Although the first recorded dictionaries date back to Sumerian times (these were bilingual dictionaries), the systematic study of dictionaries as objects of scientific interest themselves is a 20th-century enterprise, called lexicography, and largely initiated by Ladislav Zgusta. The birth of the new discipline was not without controversy, the practical dictionary-makers being sometimes accused by others of "astonishing" lack of method and critical-self reflection.