On Twitter, what is this sign "#" called?

On Twitter, the symbol "#" is called a hashtag. A hashtag combines the hash symbol with a word or phrase, usually written without spaces, so posts about the same topic can be grouped and discovered more easily. For example, #Trivia can identify posts about trivia.

The idea became strongly associated with Twitter after user Chris Messina proposed using the symbol to organize conversations in 2007. Twitter later adopted hashtags as a major discovery and navigation feature, helping users follow events, breaking news, campaigns, television discussions, and online movements.

A common mix-up is calling the symbol only a pound sign or number sign. Those names are valid in other contexts, such as telephone keypads, programming, and older American usage. "Hashtag" specifically describes the symbol when it labels a topic or keyword in social media text.

Hashtags are not limited to Twitter today. They are also used on Instagram, TikTok, Facebook, YouTube, and other platforms, although each service may rank or display them differently. A hashtag can contain letters and numbers, but spaces generally end the tag, so #ScienceFacts works while #Science Facts creates only #Science.