Because of its widespread use, hashtag was added to the Oxford English Dictionary in June 2014.The US pound sign, number sign or hash symbol "#" is often used in information technology to highlight a special meaning.Messina’s suggestion to use the hashtag was not adopted by Twitter, but the practice took off after hashtags were widely used in tweets relating to the 2007 San Diego forest fires in Southern California.According to Messina, he suggested use of the hashtag to make it easy for "lay" users to search for content and find specific relevant updates; they were for people who do not have the technological knowledge to navigate the site.
For example, on the photo-sharing service Instagram, the hashtag #bluesky allows users to find all the posts that have been tagged using that hashtag.
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.
Therefore, the hashtag "was created organically by Twitter users as a way to categorize messages." Today they are for anyone, either with or without technical knowledge, to easily impose enough annotation to be useful without needing a more formal system or adhering to many technical details.
Internationally, the hashtag became a practice of writing style for Twitter posts during the 2009–2010 Iranian election protests; Twitter users inside and outside Iran used both English- and Persian-language hashtags in communications during the events.