The word “hashtag” was added to the Oxford English Dictionary in 2014. The OED recorded it in June of that year, defining it as a word or phrase preceded by the # symbol and used on social media to find messages about the same subject.
The timing reflects how quickly the term moved from internet slang into mainstream English. Chris Messina proposed using the hash symbol to group Twitter posts in 2007, and the convention became widely visible during the San Diego wildfires later that year. Hashtags then spread through political campaigns, news events, activism, and other social networks.
The OED entry came after the practice itself was already well established, so 2014 is not the year hashtags were invented. Another frequent mistake is treating “hashtag” as the original name of the character. The symbol has older names, including hash mark, number sign, and pound sign; “hashtag” specifically describes the symbol’s use before a word or phrase for social-media classification.