Lexicographer Ben Zimmer identified Stowe Boyd’s 2007 post as the first published use of “hash tag.”
Boyd used the term in his blog post “Hash Tags = Twitter Groupings,” published on August 26, 2007. The post followed Chris Messina’s proposal that Twitter users employ the hash symbol to group related messages. Zimmer later documented and publicized the terminology’s early history as chair of the American Dialect Society’s New Words Committee.
This attribution concerns the first published use of the phrase “hash tag,” not necessarily the first use of the symbol for online grouping. The hash sign already had important roles in systems such as Internet Relay Chat, and Messina is generally credited with proposing its Twitter use. Those two origin stories are often mistakenly collapsed into one.
Twitter did not immediately adopt the idea as an official feature. Hashtags became more visible through user activity, including discussion of the 2007 San Diego forest fires, and later spread through political protests, activism, entertainment, and everyday social-media conversation. The closed-up spelling “hashtag” eventually became standard.