A hashtag is a type of meta data 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 pound sign) # (also known as number sign) in front of a word or unspaced phrase, either in the main text of a message or typically at the end. Searching for that hashtag will then present each message that has been tagged with it. A hashtag archive is consequently collected into a single stream under the same hashtag.

Social Media channels where hashtags are popular:

Hashtags are often used around large live events with broad appeal, such as the Oscars and other awards shows, the Super Bowl, the Olympics and other live sporting events, political debates, State of the Union addresses, and television shows. People using the associated hashtag for that event are participating in a global conversation about the event.

Examples of the use of hashtags in the wild:

Ian Lurie is using the hashtag #pubcon (established for a marketing conference) to broadcast to people at that conference in a crowdsourcing message for information.

Bill Gates is participating in a larger issue about the eradication of malaria. Using the hashtag #endmalaria signals that this tweet relates to that topic.