number sign

The symbol # is known as the number sign, hash, (in North America) the pound sign, and has a variety of other names. The symbol has historically been used for a wide range of purposes including the designation of an ordinal number and as a ligatured abbreviation for pounds avoirdupois – having been derived from the now-rare ℔.

Since 2007, widespread usage of the symbol to introduce metadata tags on social media platforms has led to such tags being known as 'hashtags', and from that, the symbol itself is sometimes called a hashtag.