Known by many names over the years, this symbol has gained much notoriety lately because of its use in social networking:
Yes, the hashtag, A.K.A. pound sign, A.K.A. numeral symbol. Typographers know it as the octothorp. Its name was derived from its use by cartographers as a symbol for village – eight fields around the central square. It also bears some resemblance to the sharp symbol in music – but they are not interchangeable!
So next time you’re tagging your tweet, give some thought to that lowly octothorp!