number sign
The # sign. Known as the number sign, hash, or in North America the pound sign. Sometimes also referred to as a "hashtag" due to its widespread usage in social media metatags.
The symbol has been used for a wide range of purposes, including designating a number (#2, number two), pound weight (2#, two pounds), proofreading mark indicating insert a space, the musical sharp symbol (♯) in typing, and designating social media metatags.