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A hashtag is a word or phrase preceded by a hash symbol (#) used on social media posts that categorizes the post.
The convention began on Twitter, when in October 2007, citizen journalists began using the hashtag #SanDiegoFire to tweet updates on a series of forest fires happening in San Diego.
Now users and brands use hashtags to cover serious political events (#Cairo) and entertainment topics (#MileyCyrus) alike.
The idea is this: click on a hashtag in a social media app, and you will see a stream of social media posts that use that hashtag.
Users interested in a topic will look for posts with that specific hashtag and see your posts if they too have that hashtag.
This way, users who do not follow you, or who might never have heard of you, see your posts.