Specialized dictionaries of the USA

Specialized dictionary deals with lexical units only in relation to some of their characteristic features, i.e. only in relation to their etymology, pronunciation, usage, e.g. the Longman Pronunciation Dictionary.
What is it Specialized dictionary?

A specialized dictionary is a dictionary that covers a relatively restricted set of phenomena. The definitive book on the subject includes chapters on (among others) dictionaries of:
synonyms
pronunciations
names (place names and personal names)
phrases and idioms
dialect terms
slang
quotations
etymologies

The World Book Dictionary is a two volume English dictionary published as a supplement to the World Book Encyclopedia. It was originally published in 1963 under the editorship of Clarence Barnhart, who wrote definitions for the Thorndike-Barnhart graded dictionary series for children, based on the educational works of Edward Thorndike whom Clarence Barnhart had known and worked with decades before. In some editions it was called the World Book Encyclopedia Dictionary.
World Book Dictionary

Like the encyclopedia, it is designed to be user friendly to young people, yet comprehensive enough to be useful to adults. The definitions are designed with consideration for the age at which a person usually encounters the word. Quotations or sample sentences are offered with many words. Most proper names are excluded, leaving their treatment to the encyclopedia.
The word list is based on a formula for calculating frequency of use. Originally covering about 180,000 words, it has expanded to nearly a quarter million, making it considerably larger than most dictionaries, though not of "unabridged" scope

The Double-Tongued Dictionary is an online dictionary. It catalogs a growing lexicon of undocumented or under-documented words on the fringes of English, focusing on slang, jargon, and new words.
Formerly known as the Double-Tongued Word Wrester, the dictionary strives to record terms and expressions that are omitted, or are poorly covered, in mainstream dictionaries. It also features definitions and citations of strange and unusual words or phrases such as "parergon," "epigenetics," and "bleeding deacon."
The information on this site is compiled, written and edited by lexicographer Grant Barrett.
Double-Tongued Dictionary

Urban Dictionary is a Web-based dictionary of slang words and phrases, which contains more than seven million definitions as of 2 March 2013. Submissions are regulated by volunteer editors and rated by site visitors. Time's Anita Hamilton included it on her 50 best websites of 2008 list.
The site was founded in 1999 by Aaron Peckham while he was a freshman computer science major at California Polytechnic State University. One of the first definitions on the site was "the man", referring to "the faces of the establishment put in place to 'bring us down'".
The website was referenced in a 2011 District Court complaint by ATF agents to document the meaning of the vulgarism "murk" as used in a criminal threat. It also consists of names and slang
Urban Dictionary

n the context of Urban Dictionary, "definitions" include not only literal definitions, but also descriptions. As such, "to define" a word or phrase on Urban Dictionary does not necessarily entail providing a strict definition; merely a description of some aspect of the word or phrase could suffice for inclusion in the dictionary.
Originally, Urban Dictionary was used mostly to define slang or cultural words or phrases not typically found in standard dictionaries, but it is now used to define any word or phrase. Words or phrases on Urban Dictionary may have multiple definitions, usage examples, and tags.
Visitors to Urban Dictionary may submit definitions without registering, but they must provide a valid e-mail address. Before they are included in the dictionary, all new definitions must be approved by voluntary editors. Editors are not given any guidelines to use when approving or rejecting definitions.
Content

As of March 2013, the site contains over 7 million definitions. As of April 2009, an average of 2,000 are submitted every day; the site receives approximately 15 million unique visitors per month, with 80% of users being younger than 25.тAs of February 2013, Urban Dictionary's Alexa rating is 845,with a rating of 396 in the United States and 43,559 sites linking in.
Traffic and users