A hashtag is a word or a phrase prefixed with the symbol
#. It is a form of metadata tag. Short messages on microblogging social networking services such as Twitter, Tout, identi.ca, Tumblr, Instagram, Flickr or Google+ may be tagged by including one or more with multiple words concatenated, e.g.:
- #Wikipedia is an #encyclopedia
Hashtags provide a means of grouping such messages, since one can search for the hashtag and get the set of messages that contain it.
Hashtags first appeared and were used within IRC networks to label groups and topics. They are also used to mark individual messages as relevant to a particular group, and to mark individual messages as belonging to a particular topic or "channel". Generally, channels or topics that are available across an entire IRC network are prepended with a hash symbol # (as opposed to those local to a server, which use an ampersand '&'). Hashtags' popularity grew concurrently with the rise and popularity of Twitter. It inspired Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the microblogging network. He posted the alleged first post on Twitter to include what would become the "hashtag":
|“||how do you feel about using # (pound) for groups. As in #barcamp [msg]?||”|
—Chris Messina, ("factoryjoe"), August 23, 2007
The first high-profile application of the hashtag was by San Diego, California resident Nate Ritter, who included #sandiegofire in his frequent posts on the October 2007 California wildfires hitting San Diego County. Internationally, the hashtag became a practice of writing style for Twitter posts during the 2009–2010 Iranian election protests, as both English and Persian-language hashtags became useful for Twitter users inside and outside of Iran.
The first use of the term "hash tag" for Messina's "#barcamp" notation was in a blog post by Stowe Boyd, Hash Tags = Twitter Groupings, on 26 August 2007, according to lexicographer Ben Zimmer, chair of the American Dialect Society's New Words Committee.
Beginning July 1, 2009, Twitter began to hyperlink all hashtags to search results containing all recent posts mentioning either the hashtag or the standard spelling of such words, provided that such words are written in the same order of characters. This was accentuated in 2010 with the introduction of "Trending Topics" on the Twitter front page, as highly trafficked hashtags are featured as Trending Topics.
Hashtags are mostly used as unmoderated ad-hoc discussion forums; any combination of characters led by a hash sign is a hashtag, and any hashtag, if promoted by enough individuals, can "trend" and attract more individual users to discussion using the hashtag. On Twitter, when a hashtag becomes extremely popular, it will appear in the "Trending Topics" area of a user's homepage. The trending topics can be organized by geographic area or by all of Twitter. Hashtags are neither registered nor controlled by any one user or group of users, and neither can they be "retired" from public usage, meaning that hashtags can be used in theoretical perpetuity depending upon the longevity of the word or set of characters in a written language. They also do not contain any set definitions, meaning that a single hashtag can be used for any number of purposes as espoused by those who make use of them.
Because of their loose nature, hashtags often become more recognized as associated with particular topics of discussion based upon a more specific spelling of the hashtag (e.g., "#cake" as opposed to "#thecakeisalie") that will be differentiated from a more general spelling. However, this can also make it difficult for topics to become "trending topics" because people often use different spelling or words to refer to the same topic. In order for topics to trend, there has to be a consensus, whether silent or stated, that the hashtag refers to that specific topic.
Hashtags also function as beacons in order for users to find and "follow" (subscribe) or "list" (organize into public contact lists) other users of similar interest.
Hashtags can be used on the social network Instagram, by posting pictures and hashtagging it with its subject. As an example, a photo of you and a friend posted to the social network can be hashtagged #bffl or #friends.
Hashtags are also used informally to express context around a given message, with no intent to actually categorize the message for later searching, sharing, or other reasons. This can help express humor, excitement, sadness or other contextual cues, for example, "Just found out my mom is my health teacher. #awkward" or "It's Monday!! #excited #sarcasm"
The feature has been added to other, non-short-message-oriented services, such as the user comment systems on YouTube and Gawker Media; in the case of the latter, hashtags for blog comments and directly submitted comments are used to maintain a more constant rate of user activity even when paid employees are not logged into the website. Real-time search aggregators such as the former Google Real-Time Search and Tagboard also support hashtags in syndicated posts, meaning that hashtags inserted into Twitter posts can be hyperlinked to incoming posts falling under that same hashtag; this has further enabled a view of the "river" of Twitter posts which can result from search terms or hashtags. Tagboard is a service which provides a unique, visual aggregate landing page for hashtags used on social media sites such as Twitter and Instagram.
- Diaspora software and social network
- All Gawker Media websites
- FriendFeed (since 2009)
- Sina Weibo
- Twitter (since 2009)
- YouTube (2009–2011)
- Kickstarter (2012–present, to be used in projects)
- Fetchnotes (2012–present)
- Facebook (upcoming)
- Sandglaz (2011-present)
One phenomenon specific to the Twitter system are micro-memes, which are emergent topics for which a hashtag is created, used widely for a few days, then disappears. These hashtags also show up in a number of trending topics websites, including Twitter's own front page.
The hashtag phenomenon has also been harvested for advertisement, promotion and contingency coordination. Most larger organizations will only focus on one or a small number of hashtags. However some individuals and organizations use a large number of hashtags to emphasise the broad range of concepts in which they are interested. The decision on whether to specialise in particular hashtags or promote a range depends on the marketing strategy of those involved.
Mass broadcast media 
Since 2010, television series on various television channels promote themselves through "branded" hashtag bugs. This is used as a means of promoting a backchannel of online side-discussion before, during and after an episode broadcast. Hashtag bugs appear on either corner of the screen, or they may appear at the end of an advertisement (for example, a motion picture trailer).
While personalities associated with broadcasts, such as hosts and correspondents, also promote their corporate or personal Twitter usernames in order to receive mentions and replies to posts, usage of related or "branded" hashtags alongside Twitter usernames (e.g., #edshow as well as @edshow) is increasingly encouraged as a microblogging style in order to "trend" the hashtag (and, hence, the discussion topic) in Twitter and other search engines. Broadcasters also make use of such a style in order to index select posts for live broadcast. Chloe Sladden, Twitter's director of media partnerships, identified two types of television-formatted usage of hashtags: hashtags which identify a series being broadcast (i.e. #SunnyFX) and instantaneous, "temporary" hashtags issued by television personalities to gauge topical responses from viewers during broadcasts.
Event promotion 
Organized real-world events have also made use of hashtags and ad-hoc lists for discussion and promotion among participants. Hashtags are used as beacons by event participants in order to find each other on both Twitter and, in many cases, in real life during events.
Companies and advocacy organizations have taken advantage of hashtag-based discussions for promotion of their products, services or campaigns.
Political protests and campaigns in the early 2010s, such as #OccupyWallStreet and #LibyaFeb17, have been organized around hashtags or have made extensive usage of hashtags for the promotion of discussion.
Consumer complaints 
Hashtags are often used by consumers on social media platforms in order to complain about the customer service experience with large companies. The term "bashtag" has been created to describe situations in which a corporate social media hashtag is used to criticise the company or to tell others about poor customer service. For example, in January 2012, McDonald's created the #McDStories hashtag so customers could share positive experiences about the restaurant chain. The marketing effort was cancelled after just two hours when McDonald's received numerous complaint tweets rather than the positive stories they were expecting.
In popular culture 
During the April 2011 Canadian party leader debate, then-leader of the New Democratic Party, Jack Layton, referred to Conservative Prime Minister Stephen Harper's crime policies as "a hashtag fail" (presumably "#Fail").
- Hashtags (sic) at the Twitter Fan Wiki. Retrieved on June 2, 2009.
- Tags at the identi.ca documentation. Retrieved on June 24, 2009.
- Parker, Ashley (June 10, 2011). "Twitter’s Secret Handshake". The New York Times. Retrieved July 26, 2011.
- Chris Messina ("factoryjoe") (August 23, 2007). "Twitter post".
- Nate Ritter (October 23, 2007). "Helping People Everywhere Through the San Diego Fires".
- Gabriel Snyder (Oct 15, 2009). "Anarchy in the Machine: Welcome to Gawker's Open Forums". Gawker.
- Zachary M. Seward (Oct. 15, 2009 / 8 a.m.). "Got a #tip? Gawker Media opens tag pages to masses, expecting "chaos"". Nieman Journalism Lab.
- Marco Wisniewski (Monday, February 06, 2012, 6:11 PM). "Hashtags in Orkut communities". Orkut.
- Jeff Huang, Katherine M. Thornton, Efthimis N. Efthimiadis (2010). "Conversational Tagging in Twitter". Proceedings of the 21st ACM Conference on Hypertext and Hypermedia (HT '10).
- Michael Schneider (Apr 21, 2011). "New to Your TV Screen: Twitter Hashtags". TV Guide.
- Todd Wasserman (Dec 03, 2012). "McDonald's Releases First TV Ad With Twitter Hashtag". Mashable.
- Gregory Ferenstein (April 15, 2011). "Twitter TV Hashtag Tips From Twitter's Own Expert". Fast Company.
- Akwagyiram, Alexis (17 May 2012). "Are Twitter and Facebook changing the way we complain?". BBC News. Retrieved 2012-06-12.
- Anna Mehler Paperny (Apr. 13 2011, 6:00 AM EDT). "Jack Layton's debatable 'hashtag' #fail". The Globe and Mail.
- "Canadians atwitter throughout debate". CBC News. Apr 13, 2011.