A hashtag is a metadata tag that is prefaced by the hash symbol, #. Hashtags are widely used on microblogging and photo-sharing services such as Twitter and Instagram as a form of user-generated tagging that enables cross-referencing of content sharing a subject or theme. For example, a search within Instagram for the hashtag #bluesky returns all posts that have been tagged with that hashtag. After the initial hash symbol, a hashtag may include letters, digits, and underscores.
The use of hashtags was first proposed by Chris Messina in a 2007 tweet. Messina made no attempt to patent the use because he felt "they were born of the internet, and owned by no one". Twitter initially decried hashtags as a "thing for nerds," but by the end of the decade hashtags were entrenched in the culture of the platform, and they soon emerged across Instagram, Facebook, and YouTube. In June 2014, 'hashtag' was added to the Oxford English Dictionary.
The number sign or hash symbol, '#', has long been used in information technology to highlight specific pieces of text. In 1970, the number sign was used to denote immediate address mode in the assembly language of the PDP-11 when placed next to a symbol or a number, and in 1978, Brian Kernighan and Dennis Ritchie used '#' in the C programming language to indicate special keywords that the C preprocessor had to process first. The pound sign was adopted for use within IRC (Internet Relay Chat) networks circa 1988 to label groups and topics. Channels or topics that are available across an entire IRC network are prefixed with a hash symbol # (as opposed to those local to a server, which use an ampersand '&').
How do you feel about using # (pound) for groups. As in #barcamp [msg]?-- Chris Messina, ("factoryjoe"), August 23, 2007
According to Messina, he suggested use of the hashtag to make it easy for lay users without specialized knowledge of search protocols to find specific relevant content. Therefore, the hashtag "was created organically by Twitter users as a way to categorize messages."
The first published use of the term "hash tag" was in a blog post by Stowe Boyd, "Hash Tags = Twitter Groupings," on August 26, 2007, according to lexicographer Ben Zimmer, chair of the American Dialect Society's New Words Committee.
Messina's suggestion to use the hashtag was not immediately adopted by Twitter, but the convention gained poplular acceptance when hashtags were widely used in tweets relating to the 2007 San Diego forest fires in Southern California. The hashtag gained international acceptance during the 2009-2010 Iranian election protests; Twitter users used both English- and Persian-language hashtags in communications during the events.
Beginning July 2, 2009, Twitter began to hyperlink all hashtags in tweets to Twitter search results for the hashtagged word (and for the standard spelling of commonly misspelled words). In 2010, Twitter introduced "Trending Topics" on the Twitter front page, displaying hashtags that are rapidly becoming popular, and the significance of trending hashtags has become so great that the company makes significant efforts to foil attempts to spam the trending list. During the 2010 World Cup, Twitter explicitly encouraged the use of hashtags with the temporary deployment of "hashflags", which replaced hashtags of three-letter country codes with their respective national flags.
A hashtag must begin with a hash character followed by other characters, and is terminated by a space or end of message. Some platforms may require the # to be preceded with a space. Most or all platforms that support hashtags permit the inclusion of letters (without diacritics), numerals, and underscores. Other characters may be supported on a platform-by-platform basis. Some characters, such as & are generally not supported as they may already serve other search functions. Hashtags are not case sensitive (a search for "#hashtag" will match "#HashTag" as well), but the use of embedded capitals (i.e., CamelCase) increases legibility and improves accessibility.
Languages that do not use word dividers handle hashtags differently. In China, microblogs Sina Weibo and Tencent Weibo use a double-hashtag-delimited #HashName# format, since the lack of spacing between Chinese characters necessitates a closing tag. Twitter uses a different syntax for Chinese characters and orthographies with similar spacing conventions: the hashtag contains unspaced characters, separated from preceding and following text by spaces (e.g., '? #? ?' instead of '?#') or by zero-width non-joiner characters before and after the hashtagged element, to retain a linguistically natural appearance (displaying as unspaced '#', but with invisible non-joiners delimiting the hashtag).
Some communities may limit, officially or unofficially, the number of hastags permitted on a single post.
Misuse of hashtags can lead to account suspensions. Twitter warns that adding hashtags to unrelated tweets, or repeated use of the same hashtag without adding to a conversation can filter an account from search results, or suspend the account.
In 2009, StockTwits began using ticker symbols preceded by the dollar sign (e.g., $XRX). In July 2012, Twitter began supporting the tag convention and dubbed it the "cashtag". The convention has extended to national currencies, and Cash App has implemented the cashtag to mark usernames.
Hashtags are particularly useful in unmoderated forums that lack a formal ontological organization. Hashtags help users find content similar interest. Hashtags are neither registered nor controlled by any one user or group of users. They do not contain any set definitions, meaning that a single hashtag can be used for any number of purposes, and that the widely accepted meaning of a hashtag can change with time.
Hashtags intended for discussion of a particular event tend to use an obscure wording to avoid being caught up with generic conversations on similar subjects, such as a cake festival using #cakefestival rather than simply #cake. 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. For topics to trend, there must be a consensus, whether silent or stated, that the hashtag refers to that specific topic.
Hashtags may be used informally to express context around a given message, with no intent to categorize the message for later searching, sharing, or other reasons. Hashtags may thus serve as a reflexive meta-commentary.
This can help express contextual cues or offer more depth to the information or message that appears with the hashtag. "My arms are getting darker by the minute. #toomuchfaketan". Another function of the hashtag can be used to express personal feelings and emotions. For example, with "It's Monday!! #excited #sarcasm" in which the adjectives are directly indicating the emotions of the speaker.
Verbal use of the word 'hashtag' is sometimes used in informal conversations. Use may be humorous, such as "I'm hashtag confused!" By August 2012, use of a hand gesture, sometimes called the "finger hashtag", in which the index and middle finger both hands are extended and arranged perpendicularly to form the hash, was widely documented.
Companies, businesses, and advocacy organizations have taken advantage of hashtag-based discussions for promotion of their products, services or campaigns.
Hashtags began appearing prominently in television broadcasts in the early 2010s. Broadcasters may display a hashtag as an on-screen bug, encouraging viewers to participate in a backchannel of discussion via social media prior to, during, or after the program. Television commercials have sometimes contained hashtags for similar purposes. Hashtag bugs appear on either corner of the screen, or they may appear at the end of an advertisement.
The increased usage of hashtags as brand promotion devices has been compared to the promotion of branded "keywords" by AOL in the late 1990s and early 2000s, as such keywords were also promoted at the end of television commercials and series episodes.
Organized real-world events have used hashtags and ad hoc lists for discussion and promotion among participants. Hashtags are used as beacons by event participants to find each other, both on Twitter and, in many cases, during actual physical events.
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. Hashtags have also been used to promote official events; the Finnish Ministry of Foreign Affairs officially titled the 2018 Russia-United States summit as the "#HELSINKI2018 Meeting".
Hashtags have been used to gather customer criticism of large companies. In January 2012, McDonald's created the #McDStories hashtag so that customers could share positive experiences about the restaurant chain, but the marketing effort was cancelled after two hours when critical tweets outnumbered praising ones.
The use of hashtags also reveals what feelings or sentiment an author attaches to a statement. This can range from the obvious, where a hashtag directly describes the state of mind, to the less obvious. For example, words in hashtags are the strongest predictor of whether or not a statement is sarcastic--a difficult AI problem.
Hashtags play an important role for employees and students in professional fields and education. In industry, individuals' engagement with a hashtags can provide opportunities for them develop and gain some professional knowledge in their fields.
In education, research on language teachers who engaged in the #MFLtwitterati Hashtag demonstrates the uses of hashtags for creating community and sharing teaching resources. The majority of participants reported positive impact on their teaching strategies as inspired by many ideas shared by different individuals in the Hashtag.  When used as a teaching tool, engaging hashtags has been shown to be effective in teaching language and vocabulary.
Emerging research in communication and learning demonstrates how hashtag practices influence the teaching and development of students. An analysis of eight studies that examined the use of hashtags in K-12 classrooms indicated that use of hashtags empower students to voice their ideas and help them with self-organization, emerging knowledge, and understanding space beyond place.[clarification needed] Related research demonstrated how high school students engaged hashtag communication practices developed story telling skills and cultural awareness. 
During the April 2011 Canadian party leader debate, Jack Layton, then-leader of the New Democratic Party, referred to Conservative Prime Minister Stephen Harper's crime policies as "a [sic] hashtag fail" (presumably #fail).
In 2010 Kanye West used the term "hashtag rap" to describe a style of rapping that, according to Rizoh of the Houston Press, uses "a metaphor, a pause, and a one-word punch line, often placed at the end of a rhyme". Rappers Nicki Minaj, Big Sean, Drake, and Lil Wayne are credited with the popularization of hashtag rap, while the style has been criticized by Ludacris, The Lonely Island, and various music writers.
Hashtag-centered biomedical twitter campaigns have shown to increase the reach, promotion, and visibility of healthcare-related open innovation platforms. 
if you post multiple unrelated updates to a topic using # ...