A hashtag is a word or a phrase prefixed with the symbol #. It is a form of metadata tag that helps identify a specific message or topic in microblogs. Any combination of characters led by a hash symbol can be used as a hashtag.

On Twitter, if a hashtag is promoted by enough individuals, it can trend and attract more individual users to discussion using the hashtag. When a hashtag becomes extremely popular, it will appear in the "Trending Topics" area of a user's homepage on Twitter, organized by geographic areas. 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 indefinitely depending upon the longevity of the word or set of characters. 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.