A hashtag is a word that is prefixed with the pound or number symbol, #, that denotes a label or topic. It is used in social networking conversations on Twitter, Facebook, Pinterest, Google Plus and Instagram as a means for people to categorize and find topics of interest.
The first known instance of a hashtag in social media was in 2007 when Twitter user Chris Messina used the hashtag symbol to help form groups on Twitter. Hashtags can be placed anywhere in a sentence, sometimes as part of the sentence itself or as an afterthought after the statement. Users can also create their own hashtags, sometimes making an entire phrase a hashtag such as #NewYork, #WorldSeries and #FollowFriday.