n. A word or any sequence of characters preceded by the hash sign (#), that serves to group similar Twitter posts.
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I'm not going to list all the reasons why Twitter is great. Instead, this post is to start a conversation on an endemic failing that is appearing: the repeated ‘trending’ of dull, offensive or inappropriate hashtags.
—Alexis Dormandy, “#FedUpWithCringeworthyTwitterHashtags? I am,” The Telegraph (London), May 10, 2012
They also discussed how interacting with people on Twitter is an important part of both building and joining a community. And how a hashtag is a useful way to help discover and follow topics or issues on Twitter.
—Jennifer Preston, “If Twitter Is a Work Necessity,” The New York Times, February 29, 2012
2007 (earliest)
Chris Messina has outlined (in a fairly voluminous way) a proposal for the use of hash tags (strings like “#tag”) as a way to help make sense of the noise within Twitter. …

I support the details of Chris’ spec. My sense is that tags in Twitter, as elsewhere, define shared experience of some kind, involving all those using the tag. And the use can be either actively putting a hash tag (like “#hashtag”) into a tweet, or more passively opting to follow a stream of tweets related to a tagged theme.
—Stowe Boyd, “Hash Tags = Twitter Groupings,” Stowe Boyd, August 26, 2007
Hashtags serve as unofficial categories on Twitter. People talking about, say, the World Economic Forum might include the hashtags #Davos or #WEF in their tweets. Searching Twitter for one of these hashtags returns all the recent tweets on that topic.

The first known use of a hashtag is the following tweet from August 23, 2007:

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