Urban Dictionary is a crowdsourced online dictionary for slang words and phrases, operating under the motto "Define Your World."^[2] The website was founded in 1999 by Aaron Peckham. Originally, Urban Dictionary was intended as a dictionary of slang, or cultural words or phrases, not typically found in standard dictionaries, but it is now used to define any word, event or phrase. Words or phrases on Urban Dictionary may have multiple definitions, usage examples, and tags. Some examples are "Angry Hitler"^[3] or "Russian Candy Cane."^[4] As of 2014, the dictionary had over seven million definitions, while about 2,000 new entries were being added daily.

The site was founded in 1999 by Aaron Peckham while he was a freshman computer science major at California Polytechnic State University, San Luis Obispo. He launched the site to compare urban slang used by university students in different parts of California. He had previously created a spoof version of the Ask Jeeves web search engine while studying at Cal Poly but closed the website after he received an infringement letter.^[5] He created Urban Dictionary initially as a parody of actual dictionaries, which he thought tended to be "stuffy" and "take themselves too seriously".

For the first five years, the site generated revenue without making a profit, but did not appear to incur any costs. In 2003, the website gained wider attention after a news article revealed that United Kingdom (UK) high court judges had used Urban Dictionary to assist them in a case involving two rappers (the judges unsuccessfully attempted to comprehend slang language that the rappers used).^[6][7]

By 2009, the site listed around 4 million entries and received about 2,000 new submissions per day.^[8] In April 2009, the site registered 15 million unique visitors, while 80 percent of its monthly users were younger than 25. In July 2009, Peckham explained to the New York Times that Urban Dictionary is not Wikipedia,^[9] because it doesn't attempt neutrality: "Every single word on here [Urban Dictionary] is written by someone with a point of view, with a personal experience of the word in the entry."^[10]

The website was referenced in a 2011 District Court complaint by Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) agents to document the meaning of the vulgarism "murk", as used in a criminal threat.^[11]

Over a 30-day period in March and April 2011, 67,000 people wrote 76,000 new definitions for Urban Dictionary, while 3,500 volunteer editors were registered. In an April 2011 Guardian article titled "In praise of urban dictionaries", Peckham revealed an overview of 10 rules that he had devised for the site's content: "Publish celebrity names, but reject 'real life' names. Reject nonsense, inside jokes or anything submitted in capital letters. Racial and sexual slurs are allowed, racist and sexist entries are not."^[12]

At the start of 2014, 32-year-old Peckham resided in San Francisco, U.S, and, while he did not reveal exact figures, he informed the media that the site was "stable and growing", and generated enough profit for both him and the site's maintenance. Peckham continued as the site's sole employee and maintained that he was not interested in venture funding or an IPO: "It is weird to be in Silicon Valley and want to be independent and not be on track to I.P.O. or want an acquisition ... But I think something special would be sacrificed if that were to happen." The site's audience at this stage was predominantly male and aged between 15 and 24.^[5]

As of January 5, 2014, 50% of the site's traffic was mobile, the iPhone app had been downloaded nearly three million times.^[13] Although English entries were by far the most common prior to the multilingual transition, some words from languages that have been incorporated or assimilated into English-speaking societies were published, including those from Swahili, Arabic, and the Fula languages.^[14]

In 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 intended as a dictionary of 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. Some examples include, but are not limited to, "Angry Hitler"^[3] or "Russian Candy Cane."^[4]

Visitors to Urban Dictionary may submit definitions without registering, but they must provide a valid email address.

By default, each definition is accepted or rejected based on the number of "Publish" or "Don't Publish" votes it receives from volunteer editors. The editors are not bound by any criteria for the approval or rejection of definitions. Editors previously needed a valid email address, but it is no longer required, as three options are provided for new words: "Add It!," "Keep Out!," and "I Can't Decide." However, a Facebook or Gmail account is required to post a new definition.^[15] Editors are not allowed to edit entries for spelling, wording or punctuation.

One objection about content is that the very name "Urban Dictionary" misleadingly implies that urbanites in different cities share a common vocabulary. A more serious objection is that definitions can be trumped up for the sole purpose of embarrassing people.

Although the explicit nature of many definitions on the site has led to objections, the site contains many non-explicit definitions. For example, the word "massive" is Jamaican in origin and is used to describe a group or collective. Peckham responded to the issue, stating that people may not be able to understand the meaning of such words without the aid of Urban Dictionary.

Urban Dictionary has been criticized for hosting and failing to remove offensive submissions, including ones containing racist and sexist content.^[16] For example, the abundance of racist definitions of "aboriginal" prompted a petition calling for their removal on Change.org which received over 7,000 signatures.^[17] Many of such definitions were later purged.

At the start of 2014, the dictionary had over seven million definitions, while 2,000 new entries were being added daily.^[5]

In November 2014, the Advertise page of the website stated that, on a monthly basis, Urban Dictionary averages 72 million impressions and 18 million unique readers. According to Peckham in January 2014, just under 40% of the site's traffic is international, while the site's audience was predominantly male and aged between 15 and 24.^[5][13]

In February 2019, the dictionary had over 1,500 words to define, while 4 new words were being added weekly.^[18]

As of 2013, Urban Dictionary has been used in several court cases to define slang terms not found in standard dictionaries. For example, the slang term "jack" was used to define the name the defendant used for his team, "the jack boys."^[19] Urban Dictionary was also used in a District Court complaint where a man posted a threat on a gun exchange Facebook page to "murk that cocksucker".^[11][19] The crowd-sourced dictionary was also used in a sexual harassment court case in Tennessee to define the phrase "to nut" as "to ejaculate".^[19]

In the United States, some state Departments of Motor Vehicles refer to Urban Dictionary in determining if certain license plates are appropriate or not. For example, a man in Las Vegas was allowed to keep "HOE" as his license plate after managing to convince the state, with the use of Urban Dictionary, that it meant "TAHOE", as in the vehicle made by Chevrolet, since that was already taken.^[12]

IBM had programmed Watson to use Urban Dictionary. After having all the words and definitions incorporated into Watson, it began responding to researchers' questions with profanity, leading the programmers to remove it from its memory and adding an additional filter to prevent it from swearing in the future.^[20]

In August 2019, The Malacañang Palace reacted to a definition referring to Philippine President Rodrigo Duterte, defining him as deceptive, sly, fake and other words. Presidential spokesperson Salvador Panelo said the definitions of "duterte" are "exact opposites" of the traits of President Rodrigo Duterte, and said Duterte to them means "means honest, incorruptible, politically-willed person, courageous, selfless, honest, transparent and all good things...and other synonymous terms."