While it's most famous as the social media 'hashtag,' in the US it's also called the 'pound' key on phones, and in music, it's known as the 'sharp' symbol. The emoji itself is shaped like a keyboard key.

There's a theory that it originally came from the symbol for the pound unit of weight. It later appeared on telephone buttons and in programming languages before being used on Twitter in 2007, which started the current 'hashtag' culture. It's an interesting example of a single symbol's role changing with the times.