The hashtag first appeared on Twitter in 2007. User Chris Messina proposed using the hash symbol to group related conversations, and the convention was soon adopted by Twitter users.
Messina published his proposal in August 2007, suggesting that the symbol could help people follow topics and conversations on the rapidly growing service. The first widely cited Twitter hashtag was associated with the San Diego wildfires, where users used “#sandiegofire” to organize updates and discussion. Twitter initially did not create hashtags as an official feature; users developed the practice themselves.
The format places a hash sign directly before a word or phrase, such as #science or #WorldCup. Spaces and most punctuation break a hashtag, while capitalization is often used to improve readability. Hashtags became especially useful during breaking news, elections, conferences, sports events, and online campaigns.
A common mix-up is confusing the hashtag’s 2007 debut with Twitter’s later formal support. The idea began as a user-created convention, and Twitter subsequently incorporated hashtags into search, links, trends, and discovery tools. The symbol itself had been used in computing and IRC before Twitter popularized it globally.