I remember it as the “number sign” or the “pound sign.” Jeopardy contestants might know it is called an “octothorpe.” The symbol itself goes back to Roman times, and was used extensively in the telephone industry, appearing on touch tone phone keypads as a way to allow special dialing features.
First suggested by Twitter marketing guru Chris Messina as a way to group, organize, and follow an idea in the Twitter stream, it quickly became a social media staple used not just on Twitter, but also Facebook and Instagram.
Users quick found the hashtag useful in organizing protest movements such as #BlackLivesMatter and #ArabSpring. It has been used to follow breaking and quickly evolving news stories as in #JeSuisCharlie ansd #PrayForOrlando.
And the actual term “hashtag?” It first appeared in Twitter on August 25, 2007.