A hashtag is a type of metadata tag used on social network and microblogging services, allowing users to apply dynamic, user-generated tagging that makes it possible for others to easily find messages with a specific theme or content; it allows easy, informal markup of folk taxonomy without need of any formal taxonomy or markup language. Users create and use hashtags by placing the number sign or pound sign (colloquially known as the hash character # in reference to its resemblance to several slices or chops with a sharp implement) 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 yield each message that has been tagged with it. A hashtag archive is consequently collected into a single stream under the same hashtag. For example, on the photo-sharing service Instagram, the hashtag #bluesky allows users to find all the posts that have been tagged using that hashtag.

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