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Its a tag used to talk about an especif subject on twitter.
Once all the users use it, it gets a lot easier to search subjects at the http://search.twitter.com/
#bailout I cant believe they're refusing the deal.

In this case, the hashtag is #bailout.
by danielemarques September 29, 2008
The game played by crack babies when they grow up to become old enough to leave the house (usually around 3 years old.)
Let's go play hashtag over by the poppies.
by GoingLikeSixty May 15, 2009
1) (v) to attract, through social media, attention to political issues so important to you that you are ready to go on a crusade in order to defend said issues.

2) (n): something that, prior to the advent of the social media phenomenon, was known as a 'pound key') on your phone.
I am going to hashtag ''Mitt Romney for president'', and no i won't press the hashtag-pound key- on my phone, because it doesn't serve any purpose.
by Sexydimma October 11, 2014