What is a Hashtag
A Hashtag is a word or an unspaced phrase prefixed with the number sign (“#“). It is a form of metadata tag. Words in messages on microblogging and social networking services such as Twitter, Facebook, Google+, VK or Instagram may be tagged by putting “#” before them, either as they appear in a sentence, (e.g., “New artists announced for #SXSW2014 Music Festival”) or appended to it. The term “hashtag” can also refer to the hash symbol itself.
Hashtags make it possible to group such messages, since one can search for the hashtag and get the set of messages that contain it. A hashtag is only connected to a specific medium and can therefore not be linked and connected to pictures or messages from different platforms.
Because of its widespread use, the word was added to the Oxford English Dictionary in June 2014.