A single-volume or multivolume reference work containing brief explanatory entries for terms and topics related to a specific subject or field of inquiry, usually arranged alphabetically (example: Dictionary of Neuropsychology). The entries in a dictionary are usually shorter than those contained in an encyclopedia on the same subject, but the word "dictionary" is often used in the titles of works that should more appropriately be called encyclopedias (example: Dictionary of the Middle Ages in 13 volumes). See also: biographical dictionary.A language dictionary lists the words of a language in alphabetical order, giving orthography, syllabication, pronunciation, etymology, definition, and standard usage. Some dictionaries also include synonyms, antonyms, and brief biographical and gazetteer information. In an unabridged dictionary, an attempt is made to be comprehensive in the number of terms included (example: Webster's Third New International Dictionary). An abridged dictionary provides a more limited selection of words and usually less information in each entry (Webster's New College Dictionary). In a visual dictionary, each term is illustrated. See also: desk dictionary and pocket dictionary.
Dictionaries are known to have developed from Latin glossaries as early as the 13th century. Dictionaries of the English language, limited to difficult words, were first compiled in the 17th century. Samuel Johnson's A Dictionary of the English Language, published in 1755, was the first to match in scope those produced by the academies of Continental Europe. The most famous contemporary example is the Oxford English Dictionary (1989), conceived in Britain in 1857 by the Philological Society. Some English language dictionaries are limited to a specialized vocabulary (example: Dictionary of American Slang). In libraries, at least one large printed dictionary is usually displayed open on a dictionary stand. Smaller portable editions are shelved in the reference section. Abbreviated dict. Compare with concordance and thesaurus. See also: lexicography, polyglot dictionary, and rhyming dictionary.
The ODLIS Web site is an example of an electronic dictionary. OneLook is a metadictionary that indexes English words and phrases in over 900 online dictionaries, including Merriam-Webster OnLine, with translation into other languages. Click here to connect to the Google list of online dictionaries.

From ODLIS, Online Dictionary for Libary and Information Science by Joan M. Reitz
http://lu.com/odlis/about.cfm