Hashtag

A hashtag is a type of label or metadata tag used on social network and microblogging services which makes it easier for users to find messages with a specific theme or content. Users create and use hashtags by placing the hash character (or number sign) # in front of a word or unspaced phrase, either in the main text of a message or at the end. Searching for that hashtag will then present each message that has been tagged with it.

For example, on the photo-sharing service Instagram the hashtag #bluesky allows users to find images that have been tagged as containing the sky, and #cannes2014 is a popular tag for images from the 2014 Cannes Film Festival. Hashtags can be used to collect public opinion on events and ideas at the local, corporate, or world level. For example, searching Twitter for #worldcup2014 returns many tweets from individuals around the globe about the 2014 FIFA World Cup.

Hashtags have been used for social activism. The Twitter hashtags #notallmen and #yesallwomen were used to debate misogyny after the 2014 Isla Vista killings; while the #illridewithyou hashtag was created to tag messages of support for Australian Muslims using public transport after the 2014 Sydney hostage crisis. A hashtag was also used for the Black Lives Matter movement, #BlackLivesMatter.

Because of its widespread use, hashtag was added to the Oxford English Dictionary in June 2014.

The term hashtag can also refer to the hash symbol itself when used in the context of a hashtag.

https://en.wikipedia.org/wiki/Hashtag