A hashtag is a word or a phrase prefixed with the hash symbol: #. It is a form of metadata tag, for example, short messages on microblogging and social networking services such as Twitter or Instagram may be tagged by putting ‘#’ before important words, either as they appear in a sentence, or appended to it.
Hashtags provide a means of grouping such messages, since one can search for the hashtag and get the set of messages that contain it.
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The Daily Omnivore 