A hashtag is a word or a phrase prefixed with the symbol #. It is a form of metadata tag.
Short messages on microblogging and social networking services such as Facebook, Twitter or Instagram may be tagged by putting “#” before important words, either as they appear in a sentence, (e.g. “New artists announced for #SXSW 2012 Music Festival!”) or appended to it.
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 prefixed with a hash symbol #. Hashtags’ popularity grew concurrently with the rise and popularity of Twitter.
Hashtags are mostly used as unmoderated ad hoc discussion forums; any combination of characters led by a hash symbol 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.