A hashtag, denoted by the hash sign (#) preceding a word or phrase, is a tool used on social media platforms to categorize digital content related to a specific subject. This concept, which took root on Twitter in 2009, has since been adopted into various languages such as Japanese, Korean, Chinese, and Russian. Beyond being used to highlight keywords or themes in a tweet, hashtags have served a multitude of purposes, from orchestrating disaster response initiatives to advocating for social justice and activism. They have also proven to be a powerful instrument in viral marketing techniques and professional growth. Various social media platforms, including Instagram, Pinterest, and Twitter, utilize hashtags to enhance user interaction. Organizations like the NBA and brands like McDonald’s have even integrated them into their marketing campaigns. Moreover, hashtags have created a profound cultural ripple effect, driving social campaigns like #MeToo and even shaping music genres. The communicative roles of hashtags and their influence on language and culture have begun to be explored by linguists.

Hashtag (Wikipedia)

For the group, see HashTag (group).

For the typographical symbol, see Number sign.

A hashtag is a metadata tag that is prefaced by the hash symbol, #. On social media, hashtags are used on microblogging and photo-sharing services such as X (formerly Twitter) or Tumblr as a form of user-generated tagging that enables cross-referencing of content by topic or theme. For example, a search within Instagram for the hashtag #bluesky returns all posts that have been tagged with that term. After the initial hash symbol, a hashtag may include letters, numerals, or underscores.

The use of hashtags was first proposed by American blogger and product consultant Chris Messina in a 2007 tweet. Messina made no attempt to patent the use because he felt that "they were born of the internet, and owned by no one". Hashtags became entrenched in the culture of Twitter and soon emerged across Instagram, Facebook, and YouTube. In June 2014, hashtag was added to the Oxford English Dictionary as "a word or phrase with the symbol # in front of it, used on social media websites and apps so that you can search for all messages with the same subject".