A hashtag is a keyword prefixed with the "#" symbol. You add a hash tag to a tweet to indicate the topic of the message. For example "I made a nice cake. #cooking." People on twitter use hash tags for events, ideas, conferences, TV shows, and other subjects to make it easier to find topical tweets. By using a hashtag, others can use the search tool to find everybody talking about a particular topic. Hashtags were invented by early Twitter users because there was no way (or space) to add category information or other data in the body of a tweet.
It is best not to overload your tweet with too many hashtags, use 1-3 at most.
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