In Chris Messina’s original hashtag tweet, he suggested using the # symbol for “groups.” On August 23, 2007, he asked whether Twitter users could use the pound sign to organize messages into groups, giving “#barcamp” as his example.
Messina’s idea was influenced by Internet Relay Chat, or IRC, where the # symbol commonly identified channels. He adapted that familiar convention for Twitter, which at the time had no built-in topic-labeling or group-discussion system. The proposal was meant to create a simple, user-generated way to gather related messages.
Twitter did not immediately adopt the idea officially. Users nevertheless began using hashtags during events such as the 2007 San Diego forest fires, demonstrating that the convention could make fast-moving conversations easier to follow. Twitter later began hyperlinking hashtags to search results in 2009 and introduced Trending Topics in 2010.
A common misconception is that Messina invented the # symbol itself. He proposed its modern social-media use; the symbol was already used for numbers, telephones, IRC channels, and other technical purposes.